diff --git a/docs/deployment/deployment-pipeline.md b/docs/deployment/deployment-pipeline.md
index d8eb8816..2f16099f 100644
--- a/docs/deployment/deployment-pipeline.md
+++ b/docs/deployment/deployment-pipeline.md
@@ -422,6 +422,8 @@ The ADO managed identity holds:
 
 The managed identity does **not** hold `Key Vault Secrets Officer` — it cannot write secrets. Cloud API tokens must be provisioned separately by the team managing the cloud application.
 
+The managed identity also does **not** hold `AppRoleAssignment.ReadWrite.All`. Microsoft Graph requires this explicit application permission to create app role assignments — being an owner of the enterprise app service principal is insufficient in application auth context. Granting this tenant-wide permission is not permitted under the organisation's Entra policy. Arc machine managed identity app role assignments are therefore granted manually by running `make <env> resource-group-init` under a user account with ownership of `spn-manbrs-web-api-<env>` — see [`scripts/bash/assign_arc_app_roles.sh`](../../scripts/bash/assign_arc_app_roles.sh) and the [Onboard Hospital VM runbook](./runbooks/onboard-hospital-vm.md#step-4--grant-api-access).
+
 ### Separation of environments
 
 Each environment (`dev`, `preprod`, `prod`) has its own:
diff --git a/docs/deployment/infrastructure/create-environment.md b/docs/deployment/infrastructure/create-environment.md
index aa6d7a65..0bff7bcb 100644
--- a/docs/deployment/infrastructure/create-environment.md
+++ b/docs/deployment/infrastructure/create-environment.md
@@ -52,6 +52,11 @@ az provider show --namespace Microsoft.GuestConfiguration --query registrationSt
   - Name: `spn-azure-arc-onboarding-screening-[environment]`
   - If it does not exist, raise a request with the platform team to create it
 
+- Confirm the web API enterprise application exists:
+  - Name: `spn-manbrs-web-api-[environment]`
+  - If it does not exist, raise a ServiceNow request for the platform team to create it — the `resource-group-init` Bicep deployment and the `assign_arc_app_roles.sh` script both depend on it
+  - [Form for ServiceNow](https://nhsdigitallive.service-now.com/nhs_digital?id=sc_cat_item&sys_id=28f3ab4f1bf3ca1078ac4337b04bcb78&sysparm_category=114fced51bdae1502eee65b9bd4bcbdc)
+
 ## Code
 
 - Create the configuration files in `infrastructure/environments/[environment]/`:
@@ -233,3 +238,11 @@ Terraform automatically discovers Arc machines registered in the Arc-enabled ser
 ```bash
 make [environment] terraform-apply
 ```
+
+After Terraform provisions the Hybrid Connection, re-run `resource-group-init` to assign the `Gateway.Access` app role to the new machine's managed identity:
+
+```bash
+make [environment] resource-group-init
+```
+
+> **Why is this a separate step?** The pipeline managed identity cannot create app role assignments — `AppRoleAssignment.ReadWrite.All` is required in application auth context regardless of SP ownership, and this permission is not permitted under the organisation's Entra policy. Running `resource-group-init` under a user account with ownership of `spn-manbrs-web-api-[environment]` is sufficient. Without this step the gateway will fail to authenticate against the cloud web API. See [Onboard Hospital VM — Step 4](../runbooks/onboard-hospital-vm.md#step-4--grant-api-access).
diff --git a/docs/deployment/runbooks/onboard-hospital-vm.md b/docs/deployment/runbooks/onboard-hospital-vm.md
index 62182602..6e88ee66 100644
--- a/docs/deployment/runbooks/onboard-hospital-vm.md
+++ b/docs/deployment/runbooks/onboard-hospital-vm.md
@@ -100,7 +100,24 @@ Run the ADO pipeline **Deploy Arc Infrastructure - \<env\>** manually. Terraform
 
 **Verify**: In the Azure portal, navigate to `relay-manbrs-<env>` → Hybrid Connections → `hc-gw-hull-university-teaching-hospitals-nhs-trust-rwa-01` is present.
 
-## Step 4 — Deploy the gateway application
+## Step 4 — Grant API access
+
+> [!IMPORTANT]
+> Without this step the gateway services will start but fail to authenticate against the cloud web API. The Arc machine's managed identity must be assigned the `Gateway.Access` app role on `spn-manbrs-web-api-<env>` before the first deployment.
+
+Run from a developer machine with Owner access to the enterprise application:
+
+```bash
+make <env> resource-group-init
+```
+
+This calls [`scripts/bash/assign_arc_app_roles.sh`](../../../scripts/bash/assign_arc_app_roles.sh) which discovers all Arc machines in `rg-mbsgw-<env>-uks-arc-enabled-servers` and assigns the `Gateway.Access` role to each machine's managed identity.
+
+> **Why is this manual?** The pipeline managed identity (`mi-mbsgw-<env>-adotoaz-uks`) cannot create app role assignments via the pipeline — Microsoft Graph requires `AppRoleAssignment.ReadWrite.All` in application auth context regardless of SP ownership, and this permission cannot be granted under the organisation's Entra policy. Running `resource-group-init` under a user account with ownership of the enterprise app SP is sufficient. See [Deployment Pipeline — Section 12](../deployment-pipeline.md#12-pipeline-identities-and-permissions) for details.
+
+**Verify**: In the Azure portal navigate to **Enterprise Applications → spn-manbrs-web-api-\<env\> → Users and groups**. The Arc machine (`gw-hull-university-teaching-hospitals-nhs-trust-rwa-01`) should appear with the `Gateway.Access` role.
+
+## Step 5 — Deploy the gateway application
 
 Run the ADO pipeline **Deploy Gateway - \<env\>** with:
 
@@ -115,7 +132,7 @@ The pipeline:
 2. Sends an Arc Run Command to `gw-hull-university-teaching-hospitals-nhs-trust-rwa-01` that writes `.env` and runs `deploy.ps1`
 3. Polls for completion and reports success or failure
 
-## Step 5 — Smoke test
+## Step 6 — Smoke test
 
 Run from the gateway VM or via Arc Run Command:
 
diff --git a/infrastructure/terraform/resource_group_init/bicepconfig.json b/infrastructure/terraform/resource_group_init/bicepconfig.json
new file mode 100644
index 00000000..cd15f3f3
--- /dev/null
+++ b/infrastructure/terraform/resource_group_init/bicepconfig.json
@@ -0,0 +1,5 @@
+{
+  "extensions": {
+    "microsoftGraphV1": "br:mcr.microsoft.com/bicep/extensions/microsoftgraph/v1.0:1.0.0"
+  }
+}
diff --git a/infrastructure/terraform/resource_group_init/enterpriseAppOwner.bicep b/infrastructure/terraform/resource_group_init/enterpriseAppOwner.bicep
new file mode 100644
index 00000000..a23c1a13
--- /dev/null
+++ b/infrastructure/terraform/resource_group_init/enterpriseAppOwner.bicep
@@ -0,0 +1,20 @@
+extension microsoftGraphV1
+
+param enterpriseAppClientId string
+param enterpriseAppName string
+param groupMemberIds array = []
+
+resource enterpriseAppSp 'Microsoft.Graph/servicePrincipals@v1.0' = {
+  appId: enterpriseAppClientId
+  owners: {
+    relationships: groupMemberIds
+  }
+}
+
+resource enterpriseApp 'Microsoft.Graph/applications@v1.0' = {
+  uniqueName: enterpriseAppName
+  displayName: enterpriseAppName
+  owners: {
+    relationships: groupMemberIds
+  }
+}
diff --git a/infrastructure/terraform/resource_group_init/main.bicep b/infrastructure/terraform/resource_group_init/main.bicep
index 3fade281..34cdda73 100644
--- a/infrastructure/terraform/resource_group_init/main.bicep
+++ b/infrastructure/terraform/resource_group_init/main.bicep
@@ -7,6 +7,8 @@ param storageAccountRGName string
 param storageAccountName string
 param appShortName string
 param userGroupPrincipalID string
+param enterpriseAppClientId string
+param groupMemberIds array = []
 
 var hubMap = {
   dev:                  'dev'
@@ -21,6 +23,7 @@ var privateDNSZoneRGName = 'rg-hub-${hubMap[envConfig]}-uks-private-dns-zones'
 var managedIdentityRGName = 'rg-mi-${envConfig}-uks'
 var infraResourceGroupName = 'rg-mbsgw-${envConfig}-infra'
 var keyVaultName = 'kv-mbsgw-${envConfig}-inf'
+var enterpriseAppName = 'spn-manbrs-web-api-${envConfig}'
 
 var miADOtoAZname = 'mi-${appShortName}-${envConfig}-adotoaz-uks'
 var miGHtoADOname = 'mi-${appShortName}-${envConfig}-ghtoado-uks'
@@ -207,6 +210,16 @@ resource groupNetworkContributorAssignment 'Microsoft.Authorization/roleAssignme
   }
 }
 
+module pipelineMiAppRole 'enterpriseAppOwner.bicep' = {
+  name: 'pipelineMiAppRoleAssignment'
+  scope: managedIdentityRG
+  params: {
+    enterpriseAppClientId: enterpriseAppClientId
+    enterpriseAppName: enterpriseAppName
+    groupMemberIds: groupMemberIds
+  }
+}
+
 output miPrincipalID string = managedIdentiyADOtoAZ.outputs.miPrincipalID
 output miName string = miADOtoAZname
 output keyVaultPrivateDNSZone string = keyVaultPrivateDNSZone.outputs.privateDNSZoneID
diff --git a/scripts/bash/assign_arc_app_roles.sh b/scripts/bash/assign_arc_app_roles.sh
new file mode 100755
index 00000000..9d7713de
--- /dev/null
+++ b/scripts/bash/assign_arc_app_roles.sh
@@ -0,0 +1,76 @@
+#!/usr/bin/env bash
+# Assigns the Gateway.Access app role to the managed identity of each Arc-enabled server
+# in the environment, allowing the gateway to authenticate against the web API.
+#
+# This is intentionally a manual script rather than automated via Terraform because the
+# pipeline managed identity (mi-manbrs-<env>-adotoaz-uks) lacks the AppRoleAssignment.ReadWrite.All
+# Graph application permission required to create app role assignments. In the delegated auth
+# context (user running this script), ownership of the service principal is sufficient.
+#
+# Run this script after onboarding new Arc-enabled servers to assign the Gateway.Access role
+# to their managed identities.
+#
+# Usage: ./scripts/bash/assign_arc_app_roles.sh <env> <subscription>
+# Example: ./scripts/bash/assign_arc_app_roles.sh review "Breast Screening - Manage Breast Screening - Dev"
+set -eu
+
+ENV_CONFIG="$1"
+SUBSCRIPTION="$2"
+
+enterpriseAppName="spn-manbrs-web-api-${ENV_CONFIG}"
+rgName="rg-mbsgw-${ENV_CONFIG}-uks-arc-enabled-servers"
+appRoleValue="Gateway.Access"
+
+az extension add --name connectedmachine --allow-preview true --yes 2>/dev/null || true
+
+echo "Fetching enterprise app details for: $enterpriseAppName"
+spObjectId=$(az ad sp list --filter "displayName eq '${enterpriseAppName}'" --query "[0].id" -o tsv)
+appRoleId=$(az ad sp list --filter "displayName eq '${enterpriseAppName}'" --query "[0].appRoles[?value=='${appRoleValue}'].id | [0]" -o tsv)
+
+if [ -z "$spObjectId" ]; then
+  echo "Error: Enterprise app '$enterpriseAppName' not found"
+  exit 1
+fi
+
+echo "SP object ID:          $spObjectId"
+echo "App role ($appRoleValue): $appRoleId"
+
+echo "Listing Arc machines in: $rgName"
+arcMachines=$(az connectedmachine list --resource-group "$rgName" --subscription "$SUBSCRIPTION" --query "[].name" -o tsv)
+
+if [ -z "$arcMachines" ]; then
+  echo "No Arc machines found in $rgName"
+  exit 0
+fi
+
+while IFS= read -r machine; do
+  [ -z "$machine" ] && continue
+
+  miPrincipalId=$(az connectedmachine show \
+    --resource-group "$rgName" \
+    --name "$machine" \
+    --subscription "$SUBSCRIPTION" \
+    --query "identity.principalId" -o tsv)
+
+  if [ -z "$miPrincipalId" ]; then
+    echo "  Warning: $machine has no managed identity principal ID, skipping."
+    continue
+  fi
+
+  echo "Assigning $appRoleValue to $machine (MI: $miPrincipalId)..."
+  if ! output=$(az rest --method POST \
+    --uri "https://graph.microsoft.com/v1.0/servicePrincipals/${spObjectId}/appRoleAssignedTo" \
+    --headers "Content-Type=application/json" \
+    --body "{\"principalId\": \"${miPrincipalId}\", \"resourceId\": \"${spObjectId}\", \"appRoleId\": \"${appRoleId}\"}" 2>&1); then
+    if echo "$output" | grep -q "Permission being assigned already exists"; then
+      echo "  Already assigned, skipping."
+    else
+      echo "Error: $output"
+      exit 1
+    fi
+  else
+    echo "  Successfully assigned."
+  fi
+done <<< "$arcMachines"
+
+echo "Done."
diff --git a/scripts/bash/resource_group_init.sh b/scripts/bash/resource_group_init.sh
index 22a10d93..93a4bcb7 100755
--- a/scripts/bash/resource_group_init.sh
+++ b/scripts/bash/resource_group_init.sh
@@ -22,12 +22,43 @@ fi
 
 echo "Found group Object ID: $userGroupPrincipalID"
 
+echo "Fetching members of group: $userGroupName"
+groupMemberIds=$(az ad group member list --group "$userGroupName" --query "[].id" -o json)
+echo "Found group members: $groupMemberIds"
+
+enterpriseAppName="spn-manbrs-web-api-${ENV_CONFIG}"
+echo "Fetching appId for enterprise app: $enterpriseAppName"
+enterpriseAppClientId=$(az ad sp list --filter "displayName eq '${enterpriseAppName}'" --query "[0].appId" -o tsv)
+
+if [ -z "$enterpriseAppClientId" ]; then
+  echo "Error: Enterprise app '$enterpriseAppName' not found in Entra ID"
+  exit 1
+fi
+
+echo "Found enterprise app client ID: $enterpriseAppClientId"
+
+echo "Checking uniqueName on application registration $enterpriseAppName..."
+appObjectId=$(az ad app show --id "$enterpriseAppClientId" --query id -o tsv)
+currentUniqueName=$(az rest --method GET \
+  --uri "https://graph.microsoft.com/v1.0/applications/${appObjectId}?\$select=uniqueName" \
+  --query uniqueName -o tsv 2>/dev/null || echo "")
+if [ -z "$currentUniqueName" ]; then
+  echo "Setting uniqueName to: $enterpriseAppName"
+  az rest --method PATCH \
+    --uri "https://graph.microsoft.com/v1.0/applications/${appObjectId}" \
+    --headers "Content-Type=application/json" \
+    --body "{\"uniqueName\": \"${enterpriseAppName}\"}"
+else
+  echo "uniqueName already set to: $currentUniqueName"
+fi
+
 echo "Deploy to hub subscription $HUB_SUBSCRIPTION_ID..."
 az deployment sub create --location "$REGION" --template-file infrastructure/terraform/resource_group_init/main.bicep \
   --subscription "$HUB_SUBSCRIPTION_ID" \
   --parameters enableSoftDelete="$ENABLE_SOFT_DELETE" envConfig="$ENV_CONFIG" region="$REGION" \
     storageAccountRGName="$STORAGE_ACCOUNT_RG" storageAccountName="$STORAGE_ACCOUNT_NAME" \
-    appShortName="$APP_SHORT_NAME" userGroupPrincipalID="$userGroupPrincipalID" --what-if
+    appShortName="$APP_SHORT_NAME" userGroupPrincipalID="$userGroupPrincipalID" \
+    enterpriseAppClientId="$enterpriseAppClientId" groupMemberIds="$groupMemberIds" --what-if
 
 read -r -p "Are you sure you want to execute the deployment? (y/n): " confirm
 [[ "$confirm" != "y" ]] && exit 0
@@ -36,7 +67,8 @@ output=$(az deployment sub create --location "$REGION" --template-file infrastru
   --subscription "$HUB_SUBSCRIPTION_ID" \
   --parameters enableSoftDelete="$ENABLE_SOFT_DELETE" envConfig="$ENV_CONFIG" region="$REGION" \
     storageAccountRGName="$STORAGE_ACCOUNT_RG" storageAccountName="$STORAGE_ACCOUNT_NAME" \
-    appShortName="$APP_SHORT_NAME" userGroupPrincipalID="$userGroupPrincipalID")
+    appShortName="$APP_SHORT_NAME" userGroupPrincipalID="$userGroupPrincipalID" \
+    enterpriseAppClientId="$enterpriseAppClientId" groupMemberIds="$groupMemberIds")
 
 echo "$output"
 
diff --git a/scripts/terraform/terraform.mk b/scripts/terraform/terraform.mk
index ed9f5947..e058dc47 100644
--- a/scripts/terraform/terraform.mk
+++ b/scripts/terraform/terraform.mk
@@ -21,9 +21,10 @@ ci: # Skip manual approvals when running in CI - make ci <env> <action>
 set-azure-account: # Set the Azure account for the environment - make <env> set-azure-account
 	[ "${SKIP_AZURE_LOGIN}" != "true" ] && az account set -s ${AZURE_SUBSCRIPTION} || true
 
-resource-group-init: set-azure-account get-subscription-ids # Initialise the resources required by terraform - make <env> resource-group-init
+resource-group-init: set-azure-account get-subscription-ids # Initialise resources required by terraform and assign Gateway.Access role to Arc machine managed identities - make <env> resource-group-init
 	$(eval STORAGE_ACCOUNT_NAME=sa${APP_SHORT_NAME}${ENV_CONFIG}tfstate)
 	scripts/bash/resource_group_init.sh "${REGION}" "${HUB_SUBSCRIPTION_ID}" "${ENABLE_SOFT_DELETE}" "${ENV_CONFIG}" "${STORAGE_ACCOUNT_RG}" "${STORAGE_ACCOUNT_NAME}" "${APP_SHORT_NAME}" "${ARM_SUBSCRIPTION_ID}"
+	scripts/bash/assign_arc_app_roles.sh "${ENV_CONFIG}" "${ARM_SUBSCRIPTION_ID}"
 
 get-subscription-ids: # Retrieve the hub subscription ID based on the subscription name in ${HUB_SUBSCRIPTION} - make <env> get-subscription-ids
 	$(eval HUB_SUBSCRIPTION_ID=$(shell az account show --query id --output tsv --name ${HUB_SUBSCRIPTION}))