They can't.
I mean, they can boot you out and refuse support, but academically you are allowed 10 years start to finish to complete a Fud.

I'd not recommend going over 4 but you won't fail for it.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 20 Jul 2011, 15:05, 1 reply, 15 years ago)

That's worth knowing, actually
I've been told various different things about how strict the 4-year deadline is, although I do know a guy who has cleared off and started a job elsewhere with a nearly-finished thesis who must, technically, be in his sixth, possibly seventh year now.

Though obviously, it would be kind of embarrassing if mine took that long. I intend to keep to four.
(LongJohnBaldry, Wed 20 Jul 2011, 15:13, Reply)

what might happen - absolute worst case - is if you criminally piss people off
they will refuse to support your submission, find you an external, internal etc and you'd have to do all that yourself. But the 4 year thing is almost always an internal policy, usually to do with RC funding. RCs consider a PhD a fail if not submitted within 4 years, and too many "technical fails" like that on RC money, and the RC cuts the Uni's funding.

Short answer - don't do it, but if you do, it's not an irredemable loss.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 20 Jul 2011, 15:24, Reply)