diff --git a/interface/include/tfm_its_defs.h b/interface/include/tfm_its_defs.h
index 852f8f4..a6591b1 100644
--- a/interface/include/tfm_its_defs.h
+++ b/interface/include/tfm_its_defs.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019, Arm Limited. All rights reserved.
+ * Copyright (c) 2019-2021, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -15,6 +15,12 @@
 /* Invalid UID */
 #define TFM_ITS_INVALID_UID 0
 
+/* ITS message types that distinguish ITS services. */
+#define TFM_ITS_SET                1001
+#define TFM_ITS_GET                1002
+#define TFM_ITS_GET_INFO           1003
+#define TFM_ITS_REMOVE             1004
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/interface/src/tfm_its_ipc_api.c b/interface/src/tfm_its_ipc_api.c
index 543b88f..a52197b 100644
--- a/interface/src/tfm_its_ipc_api.c
+++ b/interface/src/tfm_its_ipc_api.c
@@ -9,6 +9,7 @@
 #include "psa/internal_trusted_storage.h"
 #include "psa_manifest/sid.h"
 #include "tfm_api.h"
+#include "tfm_its_defs.h"
 
 psa_status_t psa_its_set(psa_storage_uid_t uid,
                          size_t data_length,
@@ -16,7 +17,6 @@
                          psa_storage_create_flags_t create_flags)
 {
     psa_status_t status;
-    psa_handle_t handle;
 
     psa_invec in_vec[] = {
         { .base = &uid, .len = sizeof(uid) },
@@ -24,14 +24,8 @@
         { .base = &create_flags, .len = sizeof(create_flags) }
     };
 
-    handle = psa_connect(TFM_ITS_SET_SID, TFM_ITS_SET_VERSION);
-    if (!PSA_HANDLE_IS_VALID(handle)) {
-        return PSA_ERROR_GENERIC_ERROR;
-    }
-
-    status = psa_call(handle, PSA_IPC_CALL, in_vec, IOVEC_LEN(in_vec), NULL, 0);
-
-    psa_close(handle);
+    status = psa_call(TFM_INTERNAL_TRUSTED_STORAGE_SERVICE_HANDLE, TFM_ITS_SET,
+                      in_vec, IOVEC_LEN(in_vec), NULL, 0);
 
     return status;
 }
@@ -43,7 +37,6 @@
                          size_t *p_data_length)
 {
     psa_status_t status;
-    psa_handle_t handle;
 
     psa_invec in_vec[] = {
         { .base = &uid, .len = sizeof(uid) },
@@ -58,15 +51,8 @@
         return PSA_ERROR_INVALID_ARGUMENT;
     }
 
-    handle = psa_connect(TFM_ITS_GET_SID, TFM_ITS_GET_VERSION);
-    if (!PSA_HANDLE_IS_VALID(handle)) {
-        return PSA_ERROR_GENERIC_ERROR;
-    }
-
-    status = psa_call(handle, PSA_IPC_CALL, in_vec, IOVEC_LEN(in_vec), out_vec,
-                      IOVEC_LEN(out_vec));
-
-    psa_close(handle);
+    status = psa_call(TFM_INTERNAL_TRUSTED_STORAGE_SERVICE_HANDLE, TFM_ITS_GET,
+                      in_vec, IOVEC_LEN(in_vec), out_vec, IOVEC_LEN(out_vec));
 
     *p_data_length = out_vec[0].len;
 
@@ -77,7 +63,6 @@
                               struct psa_storage_info_t *p_info)
 {
     psa_status_t status;
-    psa_handle_t handle;
 
     psa_invec in_vec[] = {
         { .base = &uid, .len = sizeof(uid) }
@@ -87,36 +72,23 @@
         { .base = p_info, .len = sizeof(*p_info) }
     };
 
-    handle = psa_connect(TFM_ITS_GET_INFO_SID, TFM_ITS_GET_INFO_VERSION);
-    if (!PSA_HANDLE_IS_VALID(handle)) {
-        return PSA_ERROR_GENERIC_ERROR;
-    }
-
-    status = psa_call(handle, PSA_IPC_CALL, in_vec, IOVEC_LEN(in_vec), out_vec,
+    status = psa_call(TFM_INTERNAL_TRUSTED_STORAGE_SERVICE_HANDLE,
+                      TFM_ITS_GET_INFO, in_vec, IOVEC_LEN(in_vec), out_vec,
                       IOVEC_LEN(out_vec));
 
-    psa_close(handle);
-
     return status;
 }
 
 psa_status_t psa_its_remove(psa_storage_uid_t uid)
 {
     psa_status_t status;
-    psa_handle_t handle;
 
     psa_invec in_vec[] = {
         { .base = &uid, .len = sizeof(uid) }
     };
 
-    handle = psa_connect(TFM_ITS_REMOVE_SID, TFM_ITS_REMOVE_VERSION);
-    if (!PSA_HANDLE_IS_VALID(handle)) {
-        return PSA_ERROR_GENERIC_ERROR;
-    }
-
-    status = psa_call(handle, PSA_IPC_CALL, in_vec, IOVEC_LEN(in_vec), NULL, 0);
-
-    psa_close(handle);
+    status = psa_call(TFM_INTERNAL_TRUSTED_STORAGE_SERVICE_HANDLE,
+                      TFM_ITS_REMOVE, in_vec, IOVEC_LEN(in_vec), NULL, 0);
 
     return status;
 }