The Liberal Democrats are under growing pressure to back Jeremy Corbyn as a caretaker prime minister to stop a no-deal Brexit after leading opposition figures and even one former Conservative minister said they were open to the idea.

With anti-no-deal groups rapidly calibrating their positions in the aftermath of Corbyn’s intervention, a split has opened up among MPs hoping to block Boris Johnson’s Brexit strategy. But while the SNP, Plaid Cymru and the Greens said they would engage with Corbyn’s offer, the Lib Dem leader, Jo Swinson, laid out a rival plan for a national unity government led by the Tory veteran Ken Clarke or Labour’s Harriet Harman.

After Swinson initially sounded a hostile note towards Corbyn’s plan, saying it was “nonsense”, the Lib Dems were widely urged to reconsider and found themselves isolated among anti-no-deal groups.

Labour’s Angela Rayner branded the Lib Dem leader “childish”, the SNP leader, Nicola Sturgeon, criticised her position as “daft”, and Caroline Lucas, the Green MP, made a personal appeal for Swinson to rethink her position.

Meanwhile Sarah Wollaston, the newest Lib Dem MP, appeared to go further than Swinson by saying backing Corbyn to avoid a no-deal Brexit would be “the lesser of two evils”.

Swinson later released a more nuanced letter to the Labour leader agreeing to meet him for talks and not ruling out the possibility of backing him as leader of a caretaker government if it would stop a no-deal Brexit.

But she challenged him to find the eight Conservative MPs necessary to make a success of his proposal for a temporary government to delay Brexit and hold a general election.

While Corbyn has seized the initiative on Brexit and forced other anti-no-deal factions to respond, he remains well short of the numbers needed to form a caretaker government, with the Lib Dems non-committal and a number of independent MPs and Tory rebels declining to back him under any circumstances.

Jeremy Corbyn urges MPs to back Labour plan to block no-deal Brexit – video

However, some of the Labour leader’s political opponents are leaving the door open to supporting him as a last resort to stop no deal. Wollaston, the former Conservative MP who had been sitting as an independent after leaving the newly formed Change UK in June, told the Guardian that while she viewed a Corbyn government as preferable to no-deal, she also viewed it as unlikely.

“Obviously, as the lesser of two evils, I would have to make a judgment and probably say: you know what, I think it would be worse to have no deal,” she said. “But let’s see what happens at that point, and ultimately what I do is beside the point. You need five or six Tories to do it, and I’m sorry but they are just not going to do it.”

Liberal Democrats may end up divided on the issue should Swinson ultimately decide to back Corbyn’s bid. The former Labour MP and strident Corbyn critic Chuka Umunna, who joined the Lib Dems this summer, is likely to oppose any option that would put Corbyn in Downing Street.

Corbyn’s offer was not dismissed out of hand by some Conservative MPs. Guto Bebb, a former Conservative minister who supports a second referendum, said: “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.”

He added: “So I think that there are other proposals that can be taken in terms of ensuring no deal is taken off the table. But I certainly take the view that a short-term Jeremy Corbyn government is less damaging than the generational damage that would be caused by a no-deal Brexit.”

Three Conservatives – Dominic Grieve, Oliver Letwin and Caroline Spelman – plus Nick Boles, a former Tory who is now independent, all agreed to meet Corbyn to discuss possible ways to block no deal. Spelman made clear there were no circumstances in which she would vote to make Corbyn prime minister.

The transport secretary, Grant Shapps, issued a stern rebuke to any Conservative MP thinking of backing the bid for a unity government in order to stop no deal, suggesting they were risking the country’s security and economic stability.

“I think it’s absolutely extraordinary that any Conservative MP considered even for one minute installing Jeremy Corbyn in Downing Street,” he said. “I just think that any Conservative should think very, very hard about doing this.”

Among the most positive towards Corbyn’s plan were Lucas and Sturgeon. The latter told an event at the Edinburgh fringe that she had argued for a progressive alternative to a Tory government at the last two elections, and “I would want us to be part of that”.

She said offering SNP support “doesn’t make me a great fan of Jeremy Corbyn … but I would always try to work to put together an alternative to a Tory government.”

Lucas was also supportive but went on to challenge Corbyn to say he would support an alternative leader if his plan failed.

She tweeted: “I welcome Corbyn’s vote of no confidence and will support his temporary government to avoid no deal (though would prefer a people’s vote before a general election). But if he can’t gain confidence of house, will he commit to support an MP from his party or another who can?”

Liz Saville-Roberts, Plaid Cymru’s Westminster leader, said her party was open to backing a unity government and it did not matter who led it, but a second referendum should be the priority. “If necessary Mr Corbyn should step aside to ensure a referendum is delivered,” she said.

Setting out her own plan, Swinson said her preferred option was legislation to request an extension of article 50 in order to hold a second referendum, with remain an option.

Her 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”.

She later released her letter to Corbyn saying she “stands ready to work with anyone to stop Boris Johnson and his hardline Brexit government pursuing no deal”, and agreeing to discussions on a “deliverable” plan.

But she said “regardless of how my party were to vote” he would still need at least eight Conservatives MPs to support his bid to take office, because a string of independents were unlikely to back him.