During a bruising debate in the House of Commons lasting almost five hours on Thursday, David Davis, the Brexit secretary, indicated he was willing to accommodate improvements to the key piece of legislation for taking Britain out of the EU in March 2019.
Dominic Grieve, a former attorney general, called the bill, which transposes EU law on to the UK statute book, an “astonishing monstrosity”, because of the breadth of the executive powers it would hand to ministers during the Brexit process.
A Conservative source said Tories on either side of the Brexit debate were now coordinating their efforts to limit these so-called Henry VIII powers, by forcing the government to make concessions or risk a damaging defeat – demonstrating the difficulties the prime minister has in getting the Brexit legislation through parliament.
In an early sign the government is preparing to bend to the anger of backbenchers, Andrea Leadsom, the leader of the House of Commons, announced that she would set aside an extra two hours to debate the bill on Monday, giving MPs until midnight to air their concerns.
The series of late-night votes will be the first major test of May’s ability to secure the parliamentary majority she needs on Brexit since the general election.
Tory rebels appear likely to back the bill at its second reading – but believe they have the numbers to force concessions when MPs debate it line-by-line, after the party conferences in October.
In a separate signal of the pressures the prime minister is likely to face from within her own party, it emerged that dozens of Eurosceptic backbenchers from the powerful European Research Group, which is funded partly through MPs’ parliamentary allowances, had signed a letter urging the government not to soften its stance on Brexit.
ERG chair Suella Fernandes, who is a junior government aide, subsequently insisted she had not signed the letter herself, merely circulated it among colleagues – and that it was supportive of the government.
She said the initiative had come from Change Britain, a lobby group that evolved out of the Vote Leave campaign, and was launched with the backing of the foreign secretary, Boris Johnson.
But the move by the backbench caucus, which coordinates parliamentary tactics and funds research backing a hard Brexit, was widely regarded at Westminster as a show of strength, after a summer in which opinion in the cabinet had appeared to be shifting towards a closer relationship with the EU after March 2019.
One Tory MP on the moderate wing of the party said: “Before the election, the ERG types were walking with a bit of a swagger and believed they were getting all of their wishes. But since then there has been a realisation across our mainstream colleagues that the PM did not get a ringing endorsement from the country for the approach set out at Lancaster House, so we do need a bit of new thinking.”
The letter, planned for publication in a Sunday newspaper but obtained by the Times and the BBC, urges the government not to “use a transition period as means of keeping the UK in the EU by stealth”.
It calls for a series of conditions to be attached to any transition deal, including a strict time-limit, the ability for Britain to withdraw unilaterally, and the ability to negotiate and sign trade deals from the outset.
“Continued membership of the single market, even as part of a transitional arrangement, would quite simply mean EU membership by another name - and we cannot allow our country to be kept in the EU by stealth. The government must respect the will of the British people, and that means leaving the single market at the same time as we leave the EU,” the letter says.
Steve Baker, who last year helped coordinate parliamentary support for leave in the referendum and is now a junior Brexit minister, was accused of rallying support for the letter. Baker, who made his dispatch box debut on Thursday, told the Guardian he could not comment on “backbench opinion”.
Fernandes, the MP for Fareham, was keen to distance herself from the letter, admitting that she had circulated it but insisting MPs had signed it “in an individual capacity”.
Jacob Rees-Mogg, who has just been elected to the House of Commons Brexit select committee, said he had signed the letter, but had done so in order to support May and Davis. “Government can be a lonely place,” he said, adding: “If I am pushing the government the way I want to go, it is a very gentle push.”
But moderate Tory MPs said the letter was a sign that hardline Brexiters in their own party were paranoid that May was backing away from the vision outlined in her Lancaster House speech in January of taking Britain out of the single market and the customs union.
The Hansard Society thinktank also criticised the bill, saying clause 17 in particular, which was condemned by Keir Starmer, would “hand the government a legislative blank cheque”.
Asked whether any minister who signed up to the letter would be in breach of their responsibilities, the prime minister’s spokeswoman said: “People have their opinions but I have set out what our intentions are.”
Damian Green, May’s de facto deputy, also sounded a conciliatory note on Thursday. “People who make reasonable points, we will listen to them and hear what they have to say … What I’m saying is we will obviously listen. It’s very important the bill will get passed,” he told Radio 4’s Today programme.
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