diff --git a/rtos/TARGET_CORTEX/mbed_rtx_conf.h b/rtos/TARGET_CORTEX/mbed_rtx_conf.h
index 91d024a260..41a5aecd17 100644
--- a/rtos/TARGET_CORTEX/mbed_rtx_conf.h
+++ b/rtos/TARGET_CORTEX/mbed_rtx_conf.h
@@ -34,11 +34,20 @@
 
 #define OS_STACK_SIZE               MBED_CONF_APP_THREAD_STACK_SIZE
 
-#define OS_TIMER_THREAD_STACK_SIZE 768
-#ifndef OS_IDLE_THREAD_STACK_SIZE
-#define OS_IDLE_THREAD_STACK_SIZE  512
+/** The timer thread's stack size can be configured by the application, if not explicitly specified, it'll default to 768 */
+#ifndef MBED_CONF_APP_TIMER_THREAD_STACK_SIZE
+#define MBED_CONF_APP_TIMER_THREAD_STACK_SIZE   768
 #endif
 
+#define OS_TIMER_THREAD_STACK_SIZE  MBED_CONF_APP_TIMER_THREAD_STACK_SIZE
+
+/** The idle thread's stack size can be configured by the application, if not explicitly specified, it'll default to 512 */
+#ifndef MBED_CONF_APP_IDLE_THREAD_STACK_SIZE
+#define MBED_CONF_APP_IDLE_THREAD_STACK_SIZE    512
+#endif
+
+#define OS_IDLE_THREAD_STACK_SIZE   MBED_CONF_APP_IDLE_THREAD_STACK_SIZE
+
 #define OS_DYNAMIC_MEM_SIZE         0
 
 #if defined(OS_TICK_FREQ) && (OS_TICK_FREQ != 1000)