The Liberal Democrat leader, Jo Swinson, has revealed the party’s proposals to stop a no-deal Brexit, including an emergency cross-party government, led by Kenneth Clarke or Harriet Harman, to replace that of Boris Johnson.

Speaking the day after Jeremy Corbyn urged opposition leaders to back a Labour plan to topple Johnson, via a no-confidence vote, and install him to lead a caretaker government before a Brexit-based general election, Swinson said she did not believe his plan was feasible.

Corbyn would not command sufficient support from rebel Tories and independent MPs to form a stable government, Swinson said in a speech, in London, in which the party introduced its newest MP, the former Tory MP Sarah Wollaston.

Instead, the Lib Dem leader argued, opposition parties should unite behind a figure such as the Conservatives’ Clarke or Labour’s Harman, respectively the longest-serving male and female MPs in the Commons, and both viewed as relative centrists in their parties.

Swinson’s refusal to back a Corbyn-led government was condemned by Labour as “childish”, with the SNP and Greens and a rebel Conservative MP also urging her to reconsider.

The Lib Dem leader said her first choice would be for parliament to pass legislation seeking to extend EU membership beyond 31 October, in order to hold a second referendum. However, she said, if this was not possible, a no-confidence vote was the next option.

Play Video 1:41 Jo Swinson says Corbyn cannot become temporary PM – video

Swinson said she had been “in contact” with Clarke and Harman, giving no other details. It is understood Harman would back the Corbyn-led temporary government as a first option, but could be open to other possibilities if this did not garner enough support.

Swinson, who was introduced by another new Lib Dem, the former Labour MP Chuka Umunna, said Johnson was “playing with our country’s future” over what she said would be a disastrous no-deal exit.

“Let me be clear – we are running out of time. But this no-deal Brexit crisis can and must be stopped,” she said.

The preferred option was legislation to request an extension of article 50 in order to hold a second referendum, with remain an option, Swinson said, but she warned that Johnson could seek to avoid that.

The next choice would be a no-confidence vote against Johnson, before “installing an emergency government with an alternative prime minister who has the confidence of the house and will stop a no-deal Brexit”.

While Corbyn would have to trigger a no-confidence vote as the leader of the opposition, she said, he had to face reality.

“We are facing a national crisis. We may need an emergency government to resolve it. But if Jeremy Corbyn truly wants that to succeed, surely even he can see that he cannot lead it.

“There is no way he can unite rebel Conservatives and independents to stop Boris Johnson. It’s not even certain he could secure all the votes of Labour MPs. This isn’t about personalities, this is about having a plan that actually works.”

Instead, she said, the temporary PM should be “a long-serving member of parliament who is respected on both sides of the house”, naming Clarke and Harman, who have respectively spent 49 and 37 years in the Commons.

Both would “not seek to lead a government in the long term”, Swinson said, and could thus be trusted.

Facebook Twitter Pinterest Opposition parties should join behind a figure such as Ken Clarke, pictured, or Harriet Harman, says Swinson. Photograph: David Levene/The Guardian

Corbyn’s plan, outlined in a letter to opposition parties on Wednesday, makes it clear the Labour frontbench considered him the only politician who could lead a caretaker government, rather than a backbench candidate.

Labour swiftly reacted to dismiss Swinson’s proposal, with the shadow education secretary, Angela Rayner, calling it “quite childish”.

Raynor told ITV News: “Jo Swinson does not get to choose who the leader of the Labour party is. Our leader is Jeremy Corbyn, and she should respect the leader of the Labour party, and respect his title as leader of the official opposition.”

Rebel Tories agree to meet Corbyn to stop no-deal Brexit Read more

Guto Bebb, a Tory MP critical of Johnson, told the BBC: “I think there are other alternatives that are open to us, but I do think that those who have said that they will do anything necessary to stop the long term damage of a no-deal exit must take seriously this type of offer.”

The SNP leader, Nicola Sturgeon, said her party was “willing to work across party lines and explore all options to stop a no-deal Brexit”. She tweeted: “Jeremy Corbyn’s suggestion is not the only possible option – but given the circumstances, nothing should be ruled out at this stage. Jo Swinson should rethink.”

Caroline Lucas, the sole Green MP, tweeted a video message appealing to Swinson to change her mind. She said: “Jo, we need to stop Boris Johnson. We need a people’s vote. Please join us in engaging with Corbyn to see if we can find a way forward.”

Asked after her speech why the Lib Dems were not supporting the idea, Swinson said it was based on realism rather than politics, adding: “What I want is a plan that works. And Jeremy Corbyn, I think, in his heart of hearts knows that he can’t command a majority in the House of Commons. He knows, everyone else knows it.”