diff --git a/rtos/source/TARGET_CORTEX/mbed_rtx_conf.h b/rtos/source/TARGET_CORTEX/mbed_rtx_conf.h
index 1679b17117..e7c6892580 100644
--- a/rtos/source/TARGET_CORTEX/mbed_rtx_conf.h
+++ b/rtos/source/TARGET_CORTEX/mbed_rtx_conf.h
@@ -82,7 +82,9 @@
 // Provide Mbed-specific instead.
 #define RTX_NO_MULTITHREAD_CLIB
 // LIBSPACE default value set for ARMCC
+#ifndef OS_THREAD_LIBSPACE_NUM
 #define OS_THREAD_LIBSPACE_NUM      4
+#endif
 
 #define OS_IDLE_THREAD_NAME         "rtx_idle"
 #define OS_TIMER_THREAD_NAME        "rtx_timer"
diff --git a/rtos/source/TARGET_CORTEX/rtx5/RTX/Config/RTX_Config.h b/rtos/source/TARGET_CORTEX/rtx5/RTX/Config/RTX_Config.h
index 3021efbc8a..0e4f9b7844 100644
--- a/rtos/source/TARGET_CORTEX/rtx5/RTX/Config/RTX_Config.h
+++ b/rtos/source/TARGET_CORTEX/rtx5/RTX/Config/RTX_Config.h
@@ -568,7 +568,9 @@
 // Number of Threads which use standard C/C++ library libspace
 // (when thread specific memory allocation is not used).
 #if (OS_THREAD_OBJ_MEM == 0)
+#ifndef OS_THREAD_LIBSPACE_NUM
 #define OS_THREAD_LIBSPACE_NUM      4
+#endif
 #else
 #define OS_THREAD_LIBSPACE_NUM      OS_THREAD_NUM
 #endif
diff --git a/tools/test_configs/CellularInterface.json b/tools/test_configs/CellularInterface.json
index 758e1037bd..e12b3cab98 100644
--- a/tools/test_configs/CellularInterface.json
+++ b/tools/test_configs/CellularInterface.json
@@ -1,4 +1,7 @@
 {
+    "macros": [
+        "OS_THREAD_LIBSPACE_NUM=5"
+    ],
     "config": {
         "echo-server-addr" : {
             "help" : "IP address of echo server",