Pro-EU Conservative backbenchers told the prime minister on Wednesday that if she “goes back” on a vow made 24 hours earlier she could yet suffer a humiliating reverse on the EU Withdrawal Bill.
On Tuesday, the government managed to buy off potential rebels with a promise to “engage positively” with their demands for MPs to have a greater say over Brexit negotiations.
Former attorney general Dominic Grieve had tabled an amendment to the bill to beef up a “meaningful vote” for MPs on the final Brexit deal.
It included a demand for the government to seek the approval of the House of Commons for its subsequent approach should no Brexit deal be agreed by the end of November. It also called on the government to cede control of the Brexit process to MPs if there is no UK-EU withdrawal agreement by 15 February 2019.
After ministers promised to compromise on his demands, Mr Grieve did not push his amendment to a vote and most pro-EU Tory MPs backed down from a potential rebellion on a similar House of Lords amendment to the bill.
However, their concerns were raised on Wednesday after a spokesman for Mrs May stated it was “a fair assessment” to say clause C of Mr Grieve’s amendment, regarding the 15 February 2019 deadline, was not up for discussion.
The spokesman added, following talks with the potential Tory rebels, that the government would bring forward an amendment to the EU Withdrawal Bill, echoing Mr Grieve’s, when it returns to the House of Lords on Monday.
Those demanding a compromise from the government on Mr Grieve’s amendment sent a warning to Mrs May as they immediately challenged Downing Street’s position on clause C.
Anna Soubry posted on Twitter: “For the avoidance of about, the PM said yesterday that clause c of Dominic Grieve’s amendment would be discussed as part of the new amendment to be tabled in the Lords.
“If the PM goes back on that there will be no agreed amendment that I can support #sortitplease.”
Fellow potential rebel Heidi Allen also said Downing Street’s position contradicted what the prime minister had told backbenchers in her parliamentary office on Tuesday.
She added that she was sure Mrs May “will honour her word”.
Despite the offer of a compromise to unhappy MPs on Tuesday, a spokesperson for the Department for Exiting the EU later drew a series of red lines the government would not cross in appeasing pro-EU Conservatives.
They said: “We have not, and will not, agree to the House of Commons binding the government’s hands in the negotiations.”
ANALYSIS: A showdown averted but not killed off
Beth Rigby, senior political correspondent
Theresa May’s Janus-faced promise on a “meaningful vote” may have averted a defeat.
But that fragile peace was already unravelling on Wednesday as two sides publicly clashed over what was or wasn’t agreed in a last-minute meeting on Tuesday afternoon.
For the Remainer rebels, there was quiet confidence on Tuesday that they had all but ruled out a no-deal Brexit after the prime minister promised to put down a new amendment to give MPs more powers over the final stages of Brexit.
The government will face an almost certain defeat if it now reneges on a promise to give MPs more of a say over the final deal.
But, for the Brexiteers, there is alarm that Mrs May has promised too much.
“Dominic Grieve was in the room. The EU will have no incentive to give us a deal because it can pass to parliament to keep us back in and extend article 50,” one senior figure on the Brexiteer side told me.
“If Grieve et al are correct, the Brexiteers can’t stomach it. It would be total, abject surrender against the Brexiteers and what the country voted for.”
No 10 was scrambling on Wednesday to reassure Brexiteers she was not giving too much away.
Downing Street insisted the key part of the Grieve amendment – to force Mrs May to “follow any direction” voted on by MPs if no deal by February – was not on the table.
That both sides have dug in so doggedly is a sign of how important – and divisive – the amendment is: it will determine whether parliament will have more power over the final stages of the Brexit deal, rather than the “take it or leave it vote” Mrs May has offered.
Remainers see it as a road map to ensuring Britain leaves with a good deal. Brexiteers fear it is a Trojan horse to frustrate Brexit and bind the prime minister’s hands.
There are mutterings from Brexiteers that a betrayal on this could be the moment they go over the top and withdraw their support from the prime minister; this is a showdown averted but not killed off.
The prime minister will be hoping that Mr Grieve et al will give her some breathing space – because the Brexiteers are making it very clear they will not.