diff --git a/src/eccodes_prototypes.h b/src/eccodes_prototypes.h
index 43ff0711f..40045b65d 100644
--- a/src/eccodes_prototypes.h
+++ b/src/eccodes_prototypes.h
@@ -1456,7 +1456,7 @@ void string_rtrim(char* s);
 void string_lrtrim(char** x, int do_left, int do_right);
 const char* extract_filename(const char* filepath);
 char** string_split(char* inputString, const char* delimiter);
-int string_to_long(const char* input, long* output);
+int string_to_long(const char* input, long* output, int strict);
 int string_ends_with(const char* str1, const char* str2);
 int string_count_char(const char* str, char c);
 const char* codes_get_product_name(ProductKind product);
diff --git a/src/grib_accessor_class_codetable.cc b/src/grib_accessor_class_codetable.cc
index e823a8659..d623f09e1 100644
--- a/src/grib_accessor_class_codetable.cc
+++ b/src/grib_accessor_class_codetable.cc
@@ -642,7 +642,7 @@ static int pack_string(grib_accessor* a, const char* buffer, size_t* len)
 {
     long lValue = 0;
     Assert(buffer);
-    if (is_number(buffer) && string_to_long(buffer, &lValue) == GRIB_SUCCESS) {
+    if (is_number(buffer) && string_to_long(buffer, &lValue, 1) == GRIB_SUCCESS) {
         // ECC-1654: If value is a pure number, just pack as long
         size_t l = 1;
         return grib_pack_long(a, &lValue, &l);
diff --git a/src/grib_accessor_class_concept.cc b/src/grib_accessor_class_concept.cc
index 20b76059d..28c09006d 100644
--- a/src/grib_accessor_class_concept.cc
+++ b/src/grib_accessor_class_concept.cc
@@ -382,7 +382,7 @@ static int grib_concept_apply(grib_accessor* a, const char* name)
                 grib_context_log(h->context, GRIB_LOG_ERROR, "concept: input handle edition=%ld, centre=%s", editionNumber, centre_s);
             }
             if (strcmp(act->name, "paramId") == 0) {
-                if (string_to_long(name, &dummy) == GRIB_SUCCESS) {
+                if (string_to_long(name, &dummy, 1) == GRIB_SUCCESS) {
                     // The paramId value is an integer. Show them the param DB
                     grib_context_log(h->context, GRIB_LOG_ERROR,
                                  "Please check the Parameter Database 'https://codes.ecmwf.int/grib/param-db/?id=%s'", name);
diff --git a/src/grib_accessor_class_long.cc b/src/grib_accessor_class_long.cc
index 6caec017a..c939189a5 100644
--- a/src/grib_accessor_class_long.cc
+++ b/src/grib_accessor_class_long.cc
@@ -257,7 +257,7 @@ static int pack_string(grib_accessor* a, const char* val, size_t* len)
     //    return pack_missing(a);
     //}
 
-    if (string_to_long(val, &v) != GRIB_SUCCESS) {
+    if (string_to_long(val, &v, 1) != GRIB_SUCCESS) {
         grib_context_log(a->context, GRIB_LOG_ERROR,
                 "Trying to pack \"%s\" as long. String cannot be converted to an integer", val);
         return GRIB_WRONG_TYPE;
diff --git a/src/grib_expression_class_functor.cc b/src/grib_expression_class_functor.cc
index 99ab0ad36..c7ffc072e 100644
--- a/src/grib_expression_class_functor.cc
+++ b/src/grib_expression_class_functor.cc
@@ -144,7 +144,7 @@ static int evaluate_long(grib_expression* g, grib_handle* h, long* lres)
             char* env = getenv(p);
             if (env) {
                 long lval = 0;
-                if (string_to_long(env, &lval) == GRIB_SUCCESS) {
+                if (string_to_long(env, &lval, 1) == GRIB_SUCCESS) {
                     *lres = lval;
                     return GRIB_SUCCESS;
                 }
diff --git a/src/grib_parse_utils.cc b/src/grib_parse_utils.cc
index e7c752f09..bbebf7df6 100644
--- a/src/grib_parse_utils.cc
+++ b/src/grib_parse_utils.cc
@@ -473,7 +473,8 @@ int grib_recompose_print(grib_handle* h, grib_accessor* observer, const char* un
                     break;
                 case '!':
                     pp = (char*)uname;
-                    if (string_to_long(uname + i + 1, &numcols) == GRIB_SUCCESS) {
+                    // Turn off strict as the input string will have a final ']' suffix
+                    if (string_to_long(uname + i + 1, &numcols, /*strict=*/0) == GRIB_SUCCESS) {
                         maxcols = (int)numcols;
                     }
                     else {
diff --git a/src/string_util.cc b/src/string_util.cc
index 65583f93e..831a4a4c4 100644
--- a/src/string_util.cc
+++ b/src/string_util.cc
@@ -124,8 +124,10 @@ char** string_split(char* inputString, const char* delimiter)
     return result;
 }
 
-/* Return GRIB_SUCCESS if can convert input to an integer, GRIB_INVALID_ARGUMENT otherwise */
-int string_to_long(const char* input, long* output)
+// Return GRIB_SUCCESS if we can convert 'input' to an integer, GRIB_INVALID_ARGUMENT otherwise.
+// If 'strict' is 1 then disallow characters at the end which are not valid digits.
+// E.g., in strict mode, "4i" will be rejected. Otherwise it will convert it to 4
+int string_to_long(const char* input, long* output, int strict)
 {
     const int base = 10;
     char* endptr;
@@ -138,11 +140,15 @@ int string_to_long(const char* input, long* output)
     val   = strtol(input, &endptr, base);
     if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN)) ||
         (errno != 0 && val == 0)) {
-        /*perror("strtol");*/
+        // perror("strtol");
         return GRIB_INVALID_ARGUMENT;
     }
     if (endptr == input) {
-        /*fprintf(stderr, "No digits were found. EXIT_FAILURE\n");*/
+        // fprintf(stderr, "No digits were found\n");
+        return GRIB_INVALID_ARGUMENT;
+    }
+    if (strict && *endptr != 0) {
+        // fprintf(stderr, "Left over characters at the end; not a pure number\n");
         return GRIB_INVALID_ARGUMENT;
     }
     *output = val;
diff --git a/tests/bufr_check_descriptors.cc b/tests/bufr_check_descriptors.cc
index 76cec0f1f..eec3a14eb 100644
--- a/tests/bufr_check_descriptors.cc
+++ b/tests/bufr_check_descriptors.cc
@@ -68,7 +68,7 @@ int main(int argc, char** argv)
             return 1;
         }
         str_code = list[0];
-        if (string_to_long(str_code, &lValue) != GRIB_SUCCESS) {
+        if (string_to_long(str_code, &lValue, 1) != GRIB_SUCCESS) {
             fprintf(stderr, "Error on line %zu: descriptor code '%s' (column 1) is not numeric.\n",
                     line_number, str_code);
             return 1;
@@ -103,17 +103,18 @@ int main(int argc, char** argv)
         str_scale = list[5];
         str_ref   = list[6];
         str_width = list[7];
-        if (string_to_long(str_scale, &lValue) != GRIB_SUCCESS) {
+        if (string_to_long(str_scale, &lValue, 1) != GRIB_SUCCESS) {
             fprintf(stderr, "Error on line %zu: descriptor scale '%s' (column 6) is not numeric.\n",
                     line_number, str_scale);
             return 1;
         }
-        if (string_to_long(str_ref, &lValue) != GRIB_SUCCESS) {
+        if (string_to_long(str_ref, &lValue, 1) != GRIB_SUCCESS) {
             fprintf(stderr, "Error on line %zu: descriptor reference '%s' (column 7) is not numeric.\n",
                     line_number, str_ref);
             return 1;
         }
-        if (string_to_long(str_width, &lValue) != GRIB_SUCCESS) {
+        // The final width column can have spaces etc at the end. So turn off strict mode
+        if (string_to_long(str_width, &lValue, /*strict=*/0) != GRIB_SUCCESS) {
             fprintf(stderr, "Error on line %zu: descriptor width '%s' (column 8) is not numeric.\n",
                     line_number, str_width);
             return 1;
diff --git a/tests/unit_tests.cc b/tests/unit_tests.cc
index 6ea0b2d15..d160b63ed 100644
--- a/tests/unit_tests.cc
+++ b/tests/unit_tests.cc
@@ -319,6 +319,31 @@ static void test_string_ends_with()
     Assert( string_ends_with("GRIB2.tmpl", " ") == 0 );
 }
 
+static void test_string_to_long()
+{
+    printf("Testing: test_string_to_long...\n");
+    long lVal = 0;
+    Assert( string_to_long("0", &lVal, 1) == GRIB_SUCCESS);
+    Assert( lVal == 0 );
+
+    Assert( string_to_long("42", &lVal, 1) == GRIB_SUCCESS);
+    Assert( lVal == 42 );
+
+    Assert( string_to_long("-1", &lVal, 1) == GRIB_SUCCESS);
+    Assert( lVal == -1 );
+    Assert( string_to_long("+999", &lVal, 1) == GRIB_SUCCESS);
+    Assert( lVal == 999 );
+
+    Assert( string_to_long("15MB", &lVal, 0) == GRIB_SUCCESS);
+    Assert( lVal == 15 );
+
+    // illegal cases
+    Assert( string_to_long("4000000000000000000000", &lVal, 1) == GRIB_INVALID_ARGUMENT);
+    Assert( string_to_long("XY", &lVal, 1) == GRIB_INVALID_ARGUMENT);
+    Assert( string_to_long("A6", &lVal, 1) == GRIB_INVALID_ARGUMENT);
+    Assert( string_to_long("5K", &lVal, 1) == GRIB_INVALID_ARGUMENT);
+}
+
 static void test_gribex_mode()
 {
     grib_context* c = grib_context_get_default();
@@ -497,6 +522,7 @@ int main(int argc, char** argv)
 
     test_trimming();
     test_string_ends_with();
+    test_string_to_long();
 
     test_get_git_sha1();
     test_get_build_date();
diff --git a/tools/grib_repair.cc b/tools/grib_repair.cc
index 00737e3e8..484bfe69f 100644
--- a/tools/grib_repair.cc
+++ b/tools/grib_repair.cc
@@ -64,7 +64,7 @@ int main(int argc, char** argv)
     sMaxNumMessages = getenv(ENV_VAR);
     if (sMaxNumMessages) {
         long lmax = 0;
-        if (string_to_long(sMaxNumMessages, &lmax) == GRIB_SUCCESS) {
+        if (string_to_long(sMaxNumMessages, &lmax, 1) == GRIB_SUCCESS) {
             MAX_NUM_MESSAGES = lmax;
         }
     }