Thx for the reply. It's not a new drill but hasn't been used frequently. Checked the bolt inside and it is totally tight. Get the feeling I might have to change the chuck for one that uses a locking key.
Sorry for delay in answering. I gave bad info in the original question. The drill is actually a Challenge Xtreme which is apparently the worst there is according to reviews. There's a bolt at the bottom of the chuck but it is locked solid. Maybe I have to consign to the refuse collection.