diff --git a/ChangeLog.d/issue665.txt b/ChangeLog.d/issue665.txt
new file mode 100644
index 0000000000..7d3da9ebce
--- /dev/null
+++ b/ChangeLog.d/issue665.txt
@@ -0,0 +1,6 @@
+Bugfix
+   * Fix a bug that caused GCM tag calculations to fail, so that data was
+     correctly encrypted but could not be authenticated. The bug was only
+     observed with GCC 10.0 to 14.2 inclusive, when compiling with -O3, and
+     running without AESNI or AESCE.
+     Fixes #665.
diff --git a/library/alignment.h b/library/alignment.h
index a17001dd91..3c107d8695 100644
--- a/library/alignment.h
+++ b/library/alignment.h
@@ -15,6 +15,7 @@
 #include <string.h>
 #include <stdlib.h>
 
+#if !defined(MBEDTLS_ALIGNMENT_DISABLE_EFFICENT_UNALIGNED_ACCESS)  //no-check-names
 /*
  * Define MBEDTLS_EFFICIENT_UNALIGNED_ACCESS for architectures where unaligned memory
  * accesses are known to be efficient.
@@ -35,7 +36,9 @@
  * device memory).
  */
 #define MBEDTLS_EFFICIENT_UNALIGNED_ACCESS
-#endif
+#endif /* __ARM_FEATURE_UNALIGNED || MBEDTLS_ARCH_IS_X86 || MBEDTLS_ARCH_IS_X64 ||
+        * MBEDTLS_PLATFORM_IS_WINDOWS_ON_ARM64 */
+#endif /* MBEDTLS_ALIGNMENT_DISABLE_EFFICENT_UNALIGNED_ACCESS */ //no-check-names
 
 #if defined(__IAR_SYSTEMS_ICC__) && \
     (defined(MBEDTLS_ARCH_IS_ARM64) || defined(MBEDTLS_ARCH_IS_ARM32) \
@@ -85,13 +88,13 @@ typedef uint64_t __packed mbedtls_uint64_unaligned_t;
  #define UINT_UNALIGNED_STRUCT
 typedef struct {
     uint16_t x;
-} __attribute__((packed)) mbedtls_uint16_unaligned_t;
+} __attribute__((packed, may_alias)) mbedtls_uint16_unaligned_t;
 typedef struct {
     uint32_t x;
-} __attribute__((packed)) mbedtls_uint32_unaligned_t;
+} __attribute__((packed, may_alias)) mbedtls_uint32_unaligned_t;
 typedef struct {
     uint64_t x;
-} __attribute__((packed)) mbedtls_uint64_unaligned_t;
+} __attribute__((packed, may_alias)) mbedtls_uint64_unaligned_t;
  #endif
 
 /*
diff --git a/tests/scripts/components-compiler.sh b/tests/scripts/components-compiler.sh
index 1eac64f54d..b764247380 100644
--- a/tests/scripts/components-compiler.sh
+++ b/tests/scripts/components-compiler.sh
@@ -172,3 +172,45 @@ component_test_zeroize () {
         done
     done
 }
+
+# This originated from an issue (https://github.com/Mbed-TLS/TF-PSA-Crypto/issues/665) found
+# in GCM when the library is built with GCC "10.0 <= version <= 14.2" on platforms other than
+# x86 and ARM64.
+component_test_tf_psa_crypto_optimized_alignment() {
+    msg "build: verify alignment with O3 optimizations in GCC"
+
+    # Disable optimizations for x86 (and ARM64) so that alignment related problems in
+    # "alignment.h" can be tested also on standard PC.
+    scripts/config.py unset MBEDTLS_AESNI_C
+    scripts/config.py unset MBEDTLS_AESCE_C
+
+    # "-O3" is the optimization level that causes issues: the compiler tries to
+    # optimize operations order and if memory dependencies are not respected
+    # (as it happens in issue tf-psa-crypto#665) this completely messes up results.
+    EXTRA_C_FLAGS="-O3"
+    # Forcedly ignore "MBEDTLS_EFFICIENT_UNALIGNED_ACCESS" on x86 so that we
+    # can test the problematic case, i.e. the case where uint64 integers are
+    # accessed through "mbedtls_uint64_unaligned_t" structs.
+    EXTRA_C_FLAGS="$EXTRA_C_FLAGS -DMBEDTLS_ALIGNMENT_DISABLE_EFFICENT_UNALIGNED_ACCESS"
+    # Adding '-g3' flag doesn't affect testing, BUT it allows to dump the generated
+    # assembly code for "gcm.o" module for inspection. To do this use the
+    # following command:
+    # > objdump -S --disassemble out_of_source_build/drivers/builtin/CMakeFiles/builtin.dir/src/gcm.c.o > gcm.s
+    # A file named "gcm.s" will be generated containing a mix of C and corresponding
+    # assembly code.
+    EXTRA_C_FLAGS="$EXTRA_C_FLAGS -g3"
+
+    make CC=gcc CFLAGS="$EXTRA_C_FLAGS"
+
+    msg "test: verify alignment with O3 optimizations in GCC"
+    make test
+}
+
+support_test_tf_psa_crypto_optimized_alignment() {
+    case $(gcc -dumpfullversion 2>/dev/null) in
+        ""|?.*) false;; # too old
+        10.*|11.*|12.*|13.*) true;;
+        14.[012].*) true;;
+        *) false;; # too recent
+    esac
+}