diff --git a/src/Http/AuthStrategy/SignedRequestStrategy.php b/src/Http/AuthStrategy/SignedRequestStrategy.php
index d27ae3c6..97327cb2 100644
--- a/src/Http/AuthStrategy/SignedRequestStrategy.php
+++ b/src/Http/AuthStrategy/SignedRequestStrategy.php
@@ -61,7 +61,7 @@ public function createQueryParams(): array
     {
         $params = [
             'nonce' => self::generateNonce(),
-            'timestamp' => (string)(round(microtime(true) * 1000)),
+            'timestamp' => sprintf('%.0F', microtime(true) * 1000),
         ];
 
         if ($this->sdkId !== null) {
diff --git a/tests/Http/AuthStrategy/SignedRequestStrategyTest.php b/tests/Http/AuthStrategy/SignedRequestStrategyTest.php
index 1d44b54b..33f432ae 100644
--- a/tests/Http/AuthStrategy/SignedRequestStrategyTest.php
+++ b/tests/Http/AuthStrategy/SignedRequestStrategyTest.php
@@ -68,6 +68,68 @@ public function shouldReturnNonceAndTimestampQueryParams()
         $this->assertNotEmpty($params['timestamp']);
     }
 
+    /**
+     * @test
+     * @covers ::createQueryParams
+     * @dataProvider lowPrecisionProvider
+     *
+     * On PHP 8.4 FPM environments (e.g. WP Engine) the `precision` ini setting
+     * can cause (string)(round(microtime(true) * 1000)) to emit scientific notation
+     * (e.g. "1.78231576830E+12"), which breaks Yoti signature verification with 401.
+     */
+    public function shouldReturnTimestampAsPlainIntegerStringUnderLowPrecision(string $precision): void
+    {
+        $strategy = new SignedRequestStrategy(PemFile::fromFilePath(TestData::PEM_FILE));
+
+        $originalPrecision = ini_get('precision');
+        ini_set('precision', $precision);
+
+        try {
+            $params = $strategy->createQueryParams();
+        } finally {
+            ini_set('precision', $originalPrecision);
+        }
+
+        $this->assertMatchesRegularExpression(
+            '/^\d+$/',
+            $params['timestamp'],
+            "Timestamp must be a plain integer string (no scientific notation) at precision={$precision}"
+        );
+    }
+
+    public function lowPrecisionProvider(): array
+    {
+        return [
+            'precision=12' => ['12'],
+            'precision=8'  => ['8'],
+        ];
+    }
+
+    /**
+     * @test
+     * @covers ::createQueryParams
+     */
+    public function shouldReturnTimestampAsUnixMilliseconds(): void
+    {
+        $strategy = new SignedRequestStrategy(PemFile::fromFilePath(TestData::PEM_FILE));
+
+        $originalPrecision = ini_get('precision');
+        ini_set('precision', '8');
+
+        try {
+            $beforeMs = (int) floor(microtime(true) * 1000);
+            $params = $strategy->createQueryParams();
+            $afterMs = (int) ceil(microtime(true) * 1000);
+        } finally {
+            ini_set('precision', $originalPrecision);
+        }
+
+        // Assert plain integer format first — (int) cast alone would mask scientific notation
+        $this->assertMatchesRegularExpression('/^\d+$/', $params['timestamp']);
+        $this->assertGreaterThanOrEqual($beforeMs, (int) $params['timestamp']);
+        $this->assertLessThanOrEqual($afterMs, (int) $params['timestamp']);
+    }
+
     /**
      * @test
      * @covers ::createQueryParams