At a “stormy” meeting, around 20 MPs confronted whips over the proposal which they fear will “tie the hands” of the UK, forcing the country out of Europe even if all necessary preparations have not been made.
The Government has been told to rewrite the proposal or face a humiliating defeat if it comes to a vote, most likely in early December.
The Independent understands ministers also face a tough battle as early as Tuesday, when a sizeable group of Tories could vote against Ms May’s Brexit plans for the EU’s Charter of Fundamental Rights.
It comes as the Government begins its push to get its main piece of Brexit legislation through a crucial stage of the parliamentary process, with MPs debating the EU withdrawal Bill into the night on Tuesday.
Ministers said Ms May’s plan to enshrine in UK law a Brexit date – 11pm GMT on 29 March 2019 – would deliver certainty about the future, but the argument has been rejected by both Tory and Labour MPs.
Former chancellor Ken Clarke said it was “not just ridiculous and unnecessary but could be positively harmful to the national interest”, while ex-attorney general Dominic Grieve has already branded it “incoherent and thoroughly stupid”.
But it has now emerged that disquiet reaches beyond the usual Conservative voices challenging Ms May’s Brexit plans.
Former minister Anna Soubry revealed details of a meeting with Chief Whip Julian Smith on Monday, saying: “It was stormy because you have got people at that meeting who have never spoken out.
“The date going into the Bill has really upset a lot of really top-quality backbench Conservative MPs.
“These are people, a lot of them ex-ministers, highly respected, and they are genuinely cross about this. There were some people there who have never rebelled and they are now talking, for the first time ever, of rebelling.”
The chairman of the Commons Foreign Affairs Committee, Tom Tugendhat, was among those who attended.
He said: “I’m very positive about the future, but this seems pointless. What difference does a week or even a month make [beyond the fixed Brexit date] after 43 years?”
Oliver Heald MP, the former solicitor-general and another who attended the meeting, said: “There were quite a lot of loyalists who never rebel who are worried about us being boxed in.”
One MP told The Independent: “There is no conceivable reason, no pragmatic reason, that this is a good idea. The numbers are growing. There are a few weeks until this will come to a head and more will come across if the Government are intransigent.
“They could either not bring this particular amendment forward or they could bring forward an adjusted amendment, which would not put the country in such an inflexible position.”
Other Tory MPs Antoinette Sandbach and Heidi Allen said another Government concession, allowing the final Brexit deal to be enshrined in a piece of legislation, would be made meaningless by fixing a Brexit date because there would likely not be enough time to pass it before the date elapses.
Shadow Brexit Secretary Sir Keir Starmer said the Government’s bid to write the date of withdrawal into law was a “desperate gimmick” from the Prime Minister in an effort to keep her party’s Eurosceptics in line.
“The Government’s amendments to their own Bill would stand in the way of an orderly transition and increase the chance of Britain crashing out of Europe without an agreement,” he said.
“Theresa May should stop pandering to the ‘no-deal’ enthusiasts in her own party and withdraw these amendments.”
But Brexit Minister Steve Baker claimed the exit date needed to be fixed to allay concerns that so-called ‘Henry VIII powers’ in the Bill are not extended infinitely, while he said others wanted it put beyond doubt when Britain would leave the EU.
The Government faces a further battle on Tuesday when another amendment to the Bill put forward by Mr Grieve is set to be debated, which would ensure that the EU’s Charter of Fundamental Rights continues to apply in UK law.
The turmoil in Parliament was reflected in uncertainty in negotiations with the EU, after a senior ally of German Chancellor Angela Merkel warned that Brexit trade talks will not be given the green light next month, as hoped by Ms May.
Ahead of a meeting with the Prime Minister on Wednesday, German MEP Manfred Weber said it “doesn’t look like” the December summit of European leaders will agree to move on to the next phase of the negotiations.
Brexit Secretary David Davis addressed financiers in London amid warnings from business leaders that unless progress is made in December, firms will be forced to make arrangements to move to other EU nations.
Mr Davis acknowledged investors needed certainty and that without a decent transition period being agreed, they may need to make decisions about their location in January.
He added: “That is why we want to agree this period as soon as the EU have a mandate to do so. Which I believe can be done, very early next year.”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies