Jo Swinson, the leader of the Liberal Democrats in the UK has said Conservative MP Ken Clarke and Labour MP Harriet Harman have both said they would be willing to lead an emergency government to halt a no-deal Brexit.

The suggestion that either Mr Clarke or Ms Harman, the House of Commons' longest-serving man and woman, could lead an anti-Brexit government was the latest sign that foes of an abrupt exit from the EU are joining forces to unseat Prime Minister Boris Johnson.

Mr Clarke later confirmed he would be willing to serve as a temporary prime minister to block a no-deal Brexit.

He told the BBC: "If it was the only way in which the plain majority of the House of Commons that is opposed to a no-deal exit could find a way forward ... I wouldn't reject it." 

Mr Clarke said he had been on holiday in Norway when Ms Swinson made the suggestion, and that it was unclear if opponents of a no-deal Brexit were sufficiently willing to compromise amongst themselves to make a temporary government viable.

Mr Johnson, who became prime minister last month after winning a vote among Conservatives for party leadership, said Britain must leave the EU on 31 October with or without a deal.

But he has a working majority of just a single seat in parliament and several of his own MPs have suggested they could vote no confidence in his government to stop a no-deal Brexit which they believe would be disastrous for the UK economy.

Labour MP Harriet Harman
Conservative MP Ken Clarke 

Anti-Brexit politicians have yet to settle on a strategy for what would happen next. 

Jeremy Corbyn, Labour Party Leader, wants a caretaker government with himself in charge and then wants to call an election.

But other opponents of a no-deal Brexit worry that Mr Corbyn, would not have enough support from across the aisle to form a government. 

Ms Swinson, whose party is pro-Remain, has proposed the idea of a caretaker government led by either Mr Clarke or Ms Harman, who are former cabinet ministers from the centrist wings of the two main parties.

Ms Swinson told BBC radio today that she had spoken to both of them and both said they would be willing to serve.

Ms Swinson said: "They put public duty first and they don't want to see a no-deal Brexit.

"If the House of Commons asks them to lead an emergency government to get our country out of this Brexit mess and to stop us driving off that cliff to a no deal, then yes they are prepared to do that."

Meanwhile, Mr Corbyn has hit back at Ms Swinson, saying that it is not up to her to decide who the next prime minister is going to be.

He told PA: "Surely she must recognise she is a leader of one of the opposition parties who are apparently opposed to this Government, and apparently prepared to support a motion of no confidence.

"I look forward to joining her in the lobbies to vote this Government down."

Mr Clarke is the longest-serving member of parliament and held senior cabinet positions under Margaret Thatcher, John Major and David Cameron. 

Ms Harman is parliament's longest-serving woman and was briefly Labour's acting leader before Mr Corbyn took charge.

With Mr Johnson staking his premiership on Brexit, the EU refusing to reopen a previously-negotiated withdrawal deal and a majority of MPs opposed to a no-deal, Britain is heading for a parliamentary showdown and constitutional crisis.

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Mr Corbyn said that by convention when a government collapses the leader of the main opposition party is called upon to form a government.

He told the BBC: "The principle is that the Labour Party is the main opposition party, we've an aspiration to go into government...We are ready to serve.

"All those people that are now making lots of noises in the media" should support Labour's motion as the best way to prevent a no-deal Brexit.

Opponents of no-deal say exiting the EU without an agreement on the transition would be a nightmare for what was once one of the West's most stable democracies. 

It would hurt global growth, send shockwaves through financial markets and weaken London’s claim to be the world’s preeminent financial centre.

Brexit supporters say there may be short-term disruption but that the economy would eventually thrive if cut free from what they cast as a doomed experiment in integration.

Labour wants a vote of no-confidence in the government shortly after parliament returns from its summer break on 3 September, Energy Minister Kwasi Kwarteng told Sky News the government would win such a vote.

Mr Kwarteng said: "I don't see Jeremy Corbyn being able to come together with the numbers, nor do I see any prospect of him leading a so-called national unity government."

"He's the most unpopular leader of the opposition we've ever had and the idea that he's going to lead a unity government I think is ridiculous."