Eight pro-Brexit lawyers, including seven Tory and DUP MPs, have outlined the key terms Theresa May must deliver to secure their support for her deal to leave the European Union.
They want to examine any agreement Attorney General Geoffrey Cox reaches with Brussels over the Northern Ireland backstop – to ensure it is temporary.
The backstop is designed to avoid a hard border with Ireland after Brexit.
It comes as a senior Brexiteer pointed to MPs’ growing mood for compromise.
Sir Graham Brady, chairman of the influential 1922 committee of backbench Conservative MPs, said: “Most MPs are in a mood to compromise, but the danger of this backstop becoming permanent is a real one and it has to be tackled.
“My conversations with senior diplomats and politicians from across Europe have given me cause for optimism that a breakthrough is near.”
He added: “We know what is needed to shift the logjam. The attorney general needs to give a legally binding guarantee that the backstop is temporary.”
The UK is due to leave the EU on 29 March, with or without a deal.
MPs last month rejected the withdrawal deal Theresa May has reached with the EU by 230 votes – the biggest defeat for a sitting government in history.
Mrs May believes the majority of the MPs who voted against her deal – including 118 Conservatives – would back it if she managed to secure changes that would prevent the UK being tied to EU customs rules indefinitely.
She has promised MPs another vote on her deal by 12 March – and if that fails, she says MPs will get a vote on whether the UK should leave without a deal, and then, by 14 March, a vote on whether Brexit should be postponed for a short period.
Attorney General Geoffrey Cox is in talks with EU officials on changes to the Irish backstop clause – seen as the sticking point for many Tories – which could tie the UK to EU customs union until a permanent trade deal has been agreed.
On Sunday, the lawyers from the European Research Group (ERG) of Brexiteer Tories, led by Jacob Rees-Mogg, announced “three tests” the government must pass to win their backing.
In private talks with Mr Cox, the ERG called for a legally binding mechanism to escape the backstop, with a clear exit route and an unambiguous rewrite of the language in the government’s legal advice, according to The Sunday Times.
The stance has been drawn up in conjunction with Northern Ireland’s Democratic Unionist Party, which Mrs May relies on to keep her in power, and which has been calling for the backstop clause to be removed from the agreement.
In an interview with the Sunday Times, one of the lawyers – Michael Tomlinson – said they must see any concession from Brussels “in black and white” and in plenty of time before the PM asks Parliament to vote on her revised deal.
Getting the backing of the ERG for her deal would be a major boost for Mrs May but would still not guarantee she could get it through Parliament.
Some Tory MPs would prefer to see the UK leave without a deal, rather than the PM’s deal, even with changes. And there are a variety of views within the ERG itself, with some leading figures taking a more hardline approach than others.
Mrs May could find herself relying on the votes of Labour MPs from Leave-voting parts of the country, who are in favour of Brexit but want guarantees from the PM that workers’ rights will not fall behind the EU after Britain’s departure.
Labour MP Caroline Flint told Sky News’ Sophy Ridge on Sunday that given the choice between “an improved deal and no deal, we should seriously consider the improved deal”.
She said as many as 30 Labour MPs felt the same way, with even more against another EU referendum.
“I think there is something like 60 or 70 Labour MPs who feel as strongly as I do against a second referendum, but also I think it’s important to recognise that many of those MPs also feel that we have to move on, we have to stop a no-deal and if there’s an improved offer on the table, then Labour should engage with that sincerely,” she added.
Labour has so far failed to get MPs to back its alternative Brexit plan, which would see the UK in a permanent customs union with the EU, and has now said it will attempt to get a new EU referendum.
Shadow chancellor John McDonnell suggested Labour frontbenchers would be expected to vote for an amendment calling for a referendum, when MPs vote on Mrs May’s deal on or before 12 March.
“I think on an issue as this we would see a whip, but also you’ve got to respect people’s views and their constituency interests as well, and the whipping arrangement will be determined in discussion in due course,” he told Sophy Ridge.