David Cameron "wouldn't last 30 seconds" if voters back Brexit in the EU referendum, according to a former Cabinet minister.
Senior Tories have warned that the campaign is pushing Tories to the brink of "civil war" and the party is facing "grave" consequences, whatever the outcome of the vote.
Feelings are running "very high" and coming to terms with the result will be a "devil of a job", Ken Clarke said.
Mr Cameron earlier this week insisted he would remain Prime Minister even if he lost the June 23 referendum.
But Mr Clarke told Radio 4's Week In Westminster: "The Prime Minister wouldn't last 30 seconds if he lost the referendum and we'd be plunged into a Conservative leadership crisis which is never a very edifying sight."
He added: "He'd be in a farcical position having campaigned for a position that's been rejected. He'd be a Prime Minister facing a parliament in which the majority of the MPs wanted to remain in the EU. 'Leave' is in a minority in the Conservative parliamentary party.
"The House of Commons as a whole is overwhelmingly in favour of staying in the European Union. And the idea that David carries on saying: 'Well despite what I've been saying in the last few weeks, I'm now going to lead a government which is going to leave the European Union and I'm going to sit down with you all and find out what it is you want to negotiate that will determine new arrangements for ourselves and our businesses and for our investors that secure a new base for us in the globalised economy', I mean it's just farcical.
"The fact is feeling is now running very high. Whichever way it goes, the party is going to have a devil of a job coming to terms. And for a prime minister to emerge in that climate, it is all very worrying. But it won't be David Cameron."
The veteran Tory said the mood in the party was heading towards the bitter divisions that left it badly damaged following clashes about the EU under Sir John Major.
Mr Clarke said: "It's dangerously close to it. And we've all got to make sure on both sides that it doesn't go back that way. The party was unelectable because it had just had the most appallingly bitter civil war and it was impossible to see how it could carry on. Now we mustn't repeat that, I agree."
Senior Tory Bernard Jenkin warned of "grave" consequences for the party, whatever the outcome of the vote.
He told the programme: "I think the Conservative Party will be in grave danger from the consequences of the result. I think a lot of people will leave the Conservative Party.
"I expect whatever emerges from the wreckage of Ukip will be more potent than before. I think these are very great dangers. And a Remain vote paradoxically makes a (Jeremy) Corbyn government somewhat more likely because the Conservative Party will be in such an unhappy state."
The warnings come as Boris Johnson continues his "Brexit blitz" across the north following a rally in Manchester where he warned the June 23 poll is the "last chance" many voters will have to decide Britain's relationship with Brussels.
London's mayor took a swipe at remain campaigners who say the EU is not perfect but claim there is no alternative.
Describing them as the "Gerald Ratners" of modern politics, the mayor said there was "not a shred of idealism" in their campaign.
Mr Cameron is among those campaigning for Britain to remain while admitting it is "not perfect".