diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 30500f4a74..e5708da7d4 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,12 @@
+2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
+	* cli/cli-cmds.c (alias_command): Check for an existing alias
+	using lookup_cmd_composition, as valid_command_p is too strict
+	and forbids aliases that are the prefix of an existing alias
+	or command.
+	* cli/cli-decode.c (lookup_cmd_composition): Ensure a prefix
+	command is properly recognised as a valid command.
+
 2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* unittests/help-doc-selftests.c: Rename to
diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index c17521b1f6..8538fadd9c 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -1694,8 +1694,29 @@ alias_command (const char *args, int from_tty)
   /* ALIAS must not exist.  */
   std::string alias_string (argv_to_string (alias_argv, alias_argc));
   alias = alias_string.c_str ();
-  if (valid_command_p (alias))
-    error (_("Alias already exists: %s"), alias);
+  {
+    cmd_list_element *alias_cmd, *prefix_cmd, *cmd;
+
+    if (lookup_cmd_composition (alias, &alias_cmd, &prefix_cmd, &cmd))
+      {
+	const char *alias_name = alias_argv[alias_argc-1];
+
+	/* If we found an existing ALIAS_CMD, check that the prefix differ or
+	   the name differ.  */
+
+	if (alias_cmd != nullptr
+	    && alias_cmd->prefix == prefix_cmd
+	    && strcmp (alias_name, alias_cmd->name) == 0)
+	  error (_("Alias already exists: %s"), alias);
+
+	/* Check ALIAS differs from the found CMD.  */
+
+	if (cmd->prefix == prefix_cmd
+	    && strcmp (alias_name, cmd->name) == 0)
+	  error (_("Alias %s is the name of an existing command"), alias);
+      }
+  }
+
 
   /* If ALIAS is one word, it is an alias for the entire COMMAND.
      Example: alias spe = set print elements
diff --git a/gdb/cli/cli-decode.c b/gdb/cli/cli-decode.c
index d951ead1c9..78b8901084 100644
--- a/gdb/cli/cli-decode.c
+++ b/gdb/cli/cli-decode.c
@@ -1843,6 +1843,8 @@ lookup_cmd_composition (const char *text,
 
   cur_list = cmdlist;
 
+  text = skip_spaces (text);
+
   while (1)
     {
       /* Go through as many command lists as we need to,
@@ -1850,9 +1852,6 @@ lookup_cmd_composition (const char *text,
 
       prev_cmd = *cmd;
 
-      while (*text == ' ' || *text == '\t')
-	(text)++;
-
       /* Identify the name of the command.  */
       len = find_command_name_length (text);
 
@@ -1861,7 +1860,7 @@ lookup_cmd_composition (const char *text,
 	return 0;
 
       /* TEXT is the start of the first command word to lookup (and
-	 it's length is len).  We copy this into a local temporary.  */
+	 it's length is LEN).  We copy this into a local temporary.  */
 
       command = (char *) alloca (len + 1);
       memcpy (command, text, len);
@@ -1890,12 +1889,14 @@ lookup_cmd_composition (const char *text,
 	    }
 	  *prefix_cmd = prev_cmd;
 	}
-      if ((*cmd)->prefixlist)
+
+      text += len;
+      text = skip_spaces (text);
+
+      if ((*cmd)->prefixlist && *text != '\0')
 	cur_list = *(*cmd)->prefixlist;
       else
 	return 1;
-
-      text += len;
     }
 }
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index dda5b68473..820af44416 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
+	* gdb.base/alias.exp: Test aliases starting with a prefix of
+	another alias.
+
 2020-05-15  Gary Benson <gbenson@redhat.com>
 
 	* gdb.base/info-os.c (main): Add return statement.
diff --git a/gdb/testsuite/gdb.base/alias.exp b/gdb/testsuite/gdb.base/alias.exp
index be78d9e936..9a9557668e 100644
--- a/gdb/testsuite/gdb.base/alias.exp
+++ b/gdb/testsuite/gdb.base/alias.exp
@@ -116,3 +116,9 @@ gdb_test "show print elements" "Limit .* is 56\[.\]" "verify 56"
 
 gdb_test_no_output "set print max-elements 57"
 gdb_test "show print elements" "Limit .* is 57\[.\]" "verify 57"
+
+# Test aliases having a common prefix.
+gdb_test_no_output "alias abcd  = backtrace"
+gdb_test_no_output "alias abcde = backtrace"
+gdb_test_no_output "alias fghij = backtrace"
+gdb_test_no_output "alias fghi  = backtrace"