diff --git a/platformio/builder/scripts/frameworks/mbed.py b/platformio/builder/scripts/frameworks/mbed.py
index 7c04af31..4f8fd57e 100644
--- a/platformio/builder/scripts/frameworks/mbed.py
+++ b/platformio/builder/scripts/frameworks/mbed.py
@@ -266,9 +266,12 @@ env.Replace(
 
 # restore external build flags
 env.ProcessFlags([
-    env.get("BOARD_OPTIONS", {}).get("build", {}).get("extra_flags"),
-    env.get("BUILD_FLAGS")
+    env.get("BOARD_OPTIONS", {}).get("build", {}).get("extra_flags")
 ])
+# remove base flags
+env.ProcessUnFlags(env.get("BUILD_UNFLAGS"))
+# apply user flags
+env.ProcessFlags([env.get("BUILD_FLAGS")])
 
 # Hook for K64F and K22F
 if board_type in ("frdm_k22f", "frdm_k64f"):