The Green MP, Caroline Lucas, has thrown down the gauntlet to 10 high-profile female politicians over blocking a no-deal Brexit, proposing a cabinet of national unity including Labour’s Emily Thornberry, the Liberal Democrat leader, Jo Swinson, and the former Conservative cabinet minister Justine Greening to seek legislation for a fresh referendum.

In an extraordinary proposal that will be viewed with scepticism by rival parties, Lucas offered to broker a deal with female MPs from all the main political parties in Westminster, as well as the SNP’s leader, Nicola Sturgeon.

Q&A What does a no-deal or WTO-rules Brexit mean? Show If the UK leaves the EU without a deal it would by default, become a “third country”, with no overarching post-Brexit plan in place and no transition period. The UK would no longer be paying into the EU budget, nor would it hand over the £39bn divorce payment. The UK would drop out of countless arrangements, pacts and treaties, covering everything from tariffs to the movement of people, foodstuffs, other goods and data, to numerous specific deals on things such as aviation, and policing and security. Without an overall withdrawal agreement each element would need to be agreed. In the immediate aftermath, without a deal the UK would trade with the EU on the default terms of the World Trade Organization (WTO), including tariffs on agricultural goods. The UK government has already indicated that it will set low or no tariffs on goods coming into the country. This would lower the price of imports – making it harder for British manufacturers to compete with foreign goods. If the UK sets the tariffs to zero on goods coming in from the EU, under WTO “most favoured nation” rules it must also offer the same zero tariffs to other countries.

WTO rules only cover goods – they do not apply to financial services, a significant part of the UK’s economy. Trading under WTO rules will also require border checks, which could cause delays at ports, and a severe challenge to the peace process in Ireland without alternative arrangements in place to avoid a hard border.

Some no-deal supporters have claimed that the UK can use article XXIV of the General Agreement on Tariffs and Trade (Gatt) to force the EU to accept a period of up to 10 years where there are no tariffs while a free trade agreement is negotiated. However, the UK cannot invoke article XXIV unilaterally – the EU would have to agree to it. In previous cases where the article has been used, the two sides had a deal in place, and it has never been used to replicate something of the scale and complexity of the EU and the UK’s trading relationship. The director general of the WTO, Roberto Azevêdo, has told Prospect magazine that “in simple factual terms in this scenario, you could expect to see the application of tariffs between the UK and EU where currently there are none”. Until some agreements are in place, a no-deal scenario will place extra overheads on UK businesses – eg the current government advice is that all drivers, including lorries and commercial vehicles, will require extra documentation to be able to drive in Europeif there is no deal. Those arguing for a “managed” no deal envisage that a range of smaller, sector-by-sector, bilateral agreements could be quickly put into place as mutual self-interest between the UK and EU to avoid introducing or to rapidly remove this kind of bureaucracy. Martin Belam

If parliament passed a motion of no confidence in Boris Johnson’s government, the plan would be for them to form a temporary cabinet, existing only to propose a Brexit referendum, likely to be remain versus no deal.

The move by Lucas is the first public offer to Labour, the Lib Dems and the SNP, as well as Tory rebels, detailing who would lead a potential government of national unity and how it could be constituted with the explicit aim of delivering a referendum.

The move is likely to have a mixed reception in Westminster from some leading figures coordinating the anti-no deal planning in parliament, several of whom are not in favour of a second referendum, including Sir Oliver Letwin and Nick Boles.

Labour has ruled out backing a replacement government not led by Jeremy Corbyn, limiting the chances that the plan could succeed. Thornberry, the shadow foreign secretary, appears unlikely to back such a move.

Lucas, who is understood to have begun talks with the women she has addressed in her open letter, sent the offer to Thornberry, Swinson, Greening and Sturgeon, as well as the Change UK MP Anna Soubry, the independent MP Heidi Allen, the SNP’s Kirsty Blackman, the independent Northern Irish MP Sylvia Hermon and Plaid Cymru’s Liz Saville Roberts.

The final MP who has been approached is Yvette Cooper, one of the leading Labour figures coordinating efforts to stop no deal, but the only one of the addressees who has not explicitly endorsed a second referendum herself.

Lucas, the former Green party leader who is her party’s only MP, wrote: “It is hard to remember a moment in my lifetime when Britain faced a greater crisis. A coup led by a small group of rightwing libertarians is all but complete, as the Vote Leave team has been reassembled and taken control of 10 Downing Street.

“They are set upon implementing the most extreme no-deal version of Brexit – and, most terrifyingly, we are running out of time to stop them.”

Lucas said she believed a cabinet of women from across the political spectrum would be best placed to set up an “emergency cabinet” which she said would “work for reconciliation” rather than fight new political battles.

“Why women? Because I believe women have shown they can bring a different perspective to crises, are able to reach out to those they disagree with and cooperate to find solutions,” she wrote.

“It was two women who began the Peace People movement during the worst of the troubles in Northern Ireland; it was two women who were key to the signing of the Paris agreement on climate; difficult, intractable problems have found the beginning of resolution thanks to the leadership of women.”

Soubry said she had been in contact with Lucas over the idea, but had some reservations: “The very serious point is that people of different political views, who might profoundly disagree over some things, need to put their differences aside and work together to stop a no-deal Brexit. That is really important.

“However, while I agree that women can do things in a different way, if you’re trying to stop no-deal Brexit then the key thing is to bring people together, not to divide them on the basis of their gender.”

But Allen said she had signed up to the plan: “Caroline is right to draw the conclusion that often it’s us women who are prepared to get people round a table and work differently.

“We are facing the precipice of no deal so we have to shed old ways of working, and fast. Women are by nature less tribal, so I’m very willing to play my part and try.”

In her letter, Lucas argued she did not accept that such an arrangement risked being an anti-democratic stitch-up.

The proposal was, she said, “not so that a small group of us decide on Britain’s future, but so that we end the dangerous pursuit of a crash-out Brexit and find a way forward which allows the British people to decide which course to take”.

Lucas said the first course of action would be to defeat Johnson in a confidence vote with the immediate aim of forming a national unity government which would negotiate an extension to article 50 in order to hold a new referendum.

That referendum would give voters the option to choose between remain and “the latest government plan”. Lucas said that could include a revised withdrawal agreement, should Johnson have produced one, but accepted it was “more likely” that the alternative would be to leave with no deal.

Lucas said those forming the new government would commit to “accept the outcome of that fair, transparent and informed vote, even if it delivers a result we do not agree with”, implying the temporary government would then dissolve.

“I believe we can make this happen,” she wrote. “I’m asking you to meet with me in the coming days, so together we can transform the conversation about Brexit. So together we can find a positive way forward, revitalise our democracy and stand up to this government’s reckless gamble with Britain’s future.”