Rofl at the who's-the-most-qualified competing on this thread. As if being qualified and experienced means being always right anyway...!
And Rofl at MrsDc7 being Peter Cheese.
There is no list of questions anywhere that by law employers are 'not allowed' to ask. But I think arguing the semantics of that isn't especially helpful. Practical advice on what to do is more helpful.
It is unlawful to discriminate, but perhaps more importantly, commercially (which is what I focus on with my clients 
), it's completely stupid to ask that question.
Because even if you do tick the box of asking all candidates, and if you completely ignore the answer, so you wouldn't be discriminating, you'd still be giving the impression of discriminating, and whether technically you're breaking a law or not, who wants to have to deal with accusations of discrimination?!
And if you have to ignore the response, and are risking challenge, there's no value in asking it at all. Asking whether someone has any commitments which will impact their ability to fulfil the role is perfectly fine. There are always ways of getting the information you need.
And in response to the OP, questions should be completely based on a person's ability to do the job. If you have a person specification, that's a good starting point. That's the list of what skills/experience/knowledge/attributes you need from the person, so use questions that ask for evidence of those things.