[cshimmin]

This is definitely not true in the US, at least in physical sciences. Typically your PhD advisor (who is obviously in the same organization and gets paid with the same funding source as the student) has almost entirely all of the say in a PhD defense. There is another faculty or two from the same department on the committee (who work on different stuff, possibly funded from somewhere else). And, IME as a mere formally, there is often another faculty from a separate academic unit (department) who's just along for the ride to give the appearance of oversight but doesn't really know what's going on. They'll ask a softball question to remind everyone they are there.

Sure, anyone in the committee can grill you as a sort of hazing ritual, but the reality is that your PhD advisor won't let you stand for defense unless you are almost 100% sure to pass it.

Source: have attended probably a dozen thesis defenses (including my own).

[pmyteh]

In the UK you can normally submit without your supervisor's approval if you insist (and have survived the programme long enough to actually have a thesis written). The actual viva will most likely be one internal academic (not your supervisor) and one external and your supervisor won't be present.

That said, it's a pretty crappy idea under nearly all circumstances -- if your supervisor doesn't think the thesis is passable and discourages you from submitting it then it's quite likely the examiners will agree with them. And getting a terminal MPhil isn't exactly a badge of honour...

[robotresearcher]

So there’s no external examiner? That’s very surprising to me, but it certainly refutes my claim.

I agree the supervisor should almost never allow a defense to take place that you won’t pass. But I can’t imagine a school passing a candidate if the external gives an unfavorable report.

I’ve also been involved in many PhD defenses, every one of which had an independent external examiner. Computer Science or closely related, not physical sciences.

[_Wintermute]

The PhD advisor is in the room and takes part in the defense in the US? Or is this just the committee?

It sounds like such a conflict to have them actually present at the defence and actively taking part.