David Cameron would be overthrown as prime minister within 30 seconds of a vote to leave the EU in the June referendum, Kenneth Clarke has said.

In a direct challenge to Cameron, who told MPs this week that he would remain in office to negotiate Britain’s exit in the event of a vote to leave the EU, the veteran pro-European said it would be “farcical” for him to continue.

The former chancellor told the Week in Westminster on BBC Radio 4: “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.”

The intervention by Clarke, whose frontbench career was revived by Cameron a year before the 2010 general election, will be seen by No 10 as particularly unhelpful. The prime minister has sought to quash questions about his own political future, as he did during the Scottish referendum, by insisting he would remain in office if he loses the EU vote. Cameron had in fact prepared a resignation speech on the eve of the Scottish referendum.

Asked during prime minister’s questions on Wednesday by the Ukip MP, Douglas Carswell, whether he would remain in office to implement a decision by the British people to leave, Cameron said: “Yes.”

But Clarke said Cameron would be quickly hounded out of office, telling Radio 4: “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.”

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Clarke, one of the few MPs who was in parliament during the referendum on Britain’s European Community (Common Market) membership in 1975, said the Tory party would have a “devil of a job” to come together, whichever way Britons vote. He added: And for a prime minister to emerge in that climate, it is all very worrying. But it won’t be David Cameron.”

He likened the atmosphere in the Tory party to the bitter battles over the Maastricht treaty during Sir John Major’s premiership in the 1990s. “It’s dangerously close to it,” said Clarke. “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.”

Bernard Jenkin, the senior Tory backbencher who supports the leave campaign, warned the party would be in “grave danger” if the UK voted to remain in the EU. Jenkin said: “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 Corbyn government somewhat more likely because the Conservative party will be in such an unhappy state.

“Certainly our voters will vote ‘leave’. The vast majority of our activists will vote ‘leave’. And under these circumstances, the Conservative party will be far more governable and leadable, and Ukip will go away, if we have a ‘leave’ vote.”