Theresa May has been forced to reassure jittery Brexiters on the Conservative backbenches as her 11-member Brexit inner cabinet prepared to assemble for an eight-hour awayday to thrash out a deal on Britain’s future relationship with the EU.

After another day in which Tory differences over Brexit were exposed, ministers were summoned to the prime minister’s country retreat of Chequers for talks scheduled to go on until 10pm on Thursday.

May’s challenge in reconciling the opposing views in her party was underlined when Downing Street moved swiftly to deny that the government was seeking an open-ended transition period after Britain leaves the EU in March 2019.

Senior pro-Brexit MPs had sought reassurances from cabinet ministers after leaked negotiating guidelines said: “The UK believes the period’s duration should be determined simply by how long it will take to prepare and implement the new processes and new systems that will underpin the future relationship.

“The UK agrees this points to a period of around two years, but wishes to discuss with the EU the assessment that supports its proposed end date.”

During the transition period, the UK will effectively remain a member of the single market and customs union under the jurisdiction of the European court of justice, but without any say in EU rules.

Sources at the Department for Exiting the European Union (DExEU) insisted officials were simply trying to force the European commission to justify its insistence that the transition period should end 21 months after Brexit day on 29 March next year.

In practice, Britain also fears that it could end up paying more for a longer transition, which would extend into the EU’s next budget period. One cabinet minister said: “It’s only an issue of three months in reality and will very likely prove too expensive to change.”

But at Westminster on Wednesday, nervous pro-Brexit MPs, who fear civil servants want to bounce the government into an unlimited transition that could leave Britain effectively remaining in the EU, challenged the government about the leaked document.

One senior Brexiter told the Guardian he had been informed by three cabinet ministers that the paper was only a negotiating position and had not been formally adopted by the cabinet.



One cabinet member told MPs the document, which was later published by the government, was a “draft of a draft”.



But a Whitehall source claimed it had been agreed by May’s Brexit subcommittee, which includes some of the most senior cabinet figures, at a meeting in January.



Northern Ireland is expected to be a sticking point on Thursday. Two key paragraphs in the agreement May struck with the EU in December said that if the final deal does not obviate the need for border checks, and a separate solution cannot be found for Northern Ireland, the UK will “maintain full alignment with those rules of the internal market and the customs union, which, now or in the future, support north-south cooperation”.

Pro-Brexit ministers are concerned that formalising this pledge will effectively commit the government to remaining in the customs union, which would prevent Liam Fox, the international trade secretary, from striking independent trade deals.

Jacob Rees-Mogg, the chairman of the European Research Group of Conservative pro-Brexit MPs, used an article in the Daily Telegraph to criticise the DExEU paper, saying the language used on the duration of the transition “translates from bureaucratese into English: ‘We must remain’”.



“When we leave the EU on 29 March next year, we need not continue to behave as if we were still a member. That would make us a vassal state and there have been no vassals in this country since the era of the Plantagenets,” he wrote.

Rees-Mogg played down the challenge presented by the Irish border problem.



Jeremy Corbyn suggested on Tuesday that having “a customs union” with the EU was the only way to avoid a hard border between Northern Ireland and the Irish Republic. The government faces a rebellion over the issue when its trade bill comes to the House of Commons in the coming weeks.



The sensitivities of the Northern Ireland situation were underlined after the Democratic Unionist party and Sinn Féin emerged from talks with May at No 10 to say little progress had been made over the issue of restoring power-sharing at Stormont.

Mary Lou McDonald, the Sinn Féin leader, said: “Today it was clear that the British government has no plan. We made it clear to Theresa May that the collapse of the talks by the DUP cannot be an excuse for the continued denial of rights to citizens in the north or the refusal to implement previous agreements.

“While the talks process has been collapsed, standing still is not an option for the British government in particular.”

There was no mention in the government’s transition negotiating guidelines of the rights of EU citizens. May’s spokesman insisted there was no change in the government’s position, which is that EU citizens arriving after Brexit day in March 2019 will be arriving in “different circumstances” and cannot expect to have the same rights as those who came beforehand.

But a compromise appears likely, with Brussels determined not to budge. Diane Abbott, the shadow home secretary, said it would be impractical for the Home Office to have two different classes of EU arrivals and treat them differently.

“We don’t want a two-tier system for EU nationals, and apart from anything else, it would lead to a complexity that I doubt the immigration services could actually handle,” she said.

England Expects. Chequers meeting really MUST agree way ahead https://t.co/Qxmh87JcOk more time after this to determine clear credible negotiating position ahead of European Council #comeonforGodssakedoit — Nicholas Soames (@NSoames) February 21, 2018

Paul Blomfield, the shadow Brexit minister, said: “The time for meaningless soundbites and conflicting statements is over. By the end of this latest ‘crunch meeting’, we need a clear vision of what the government wants our future relationship with the EU to look like, and one which meets the needs of the country rather than the demands of her rebellious backbenchers.

“If the prime minister can’t achieve this, it proves she’s not fit to lead the negotiations.”