Labour's shadow Brexit secretary has written to the Prime Minister urging her to back his party's amendment to the EU (Withdrawal) bill.
Sir Keir Starmer says unless the Government backs the Labour amendment to allow the single market to continue operating for another two years after Brexit, Theresa May's pledge to implement a transitional arrangement will be impossible.
Sir Keir told Sky News that "everybody" agrees that a transitional deal will be necessary because a trade deal with the EU will not be in place by Brexit in 2019.
But he said Mrs May's offer to the EU of a two-year transitional period, made during her speech in Florence in September, would not be possible because it "extinguishes" Britain's subjection to the European Court of Justice – the court that underpins the framework of the single market.
It comes after repeated calls from senior hardline Brexiteer Conservatives for the Government to walk away from an EU deal, or at least prepare for no deal.
It was revealed on Sunday that Boris Johnson and Michael Gove became the latest to do so, joining forces to write to Mrs May, calling on her to ensure all members of her senior team were behind Brexit and warning of the consequences of failing to plan.
Sir Keir called on Mrs May to act in the "national interest", by supporting Labour's proposals for transitional arrangements, or at least to table an amendment that proposes the same arrangements.
He said: "Step back, think about this amendment, table the self-same amendment if you want, for heaven's sake, act now in the national interest, not in party interest."
On Monday morning, Mrs May was preparing to meet European business leaders to discuss the future of UK-EU trade after Brexit.
The Prime Minister will attempt to win support from European firms for her goal of moving the negotiations on to trade talks.
A former Conservative Government member, pro-remain Dominic Grieve, criticised the Government's own Brexit bill amendment, tabled last week, which would set the date for leaving the EU as 29 March 2019.
The amendments will be debated as the landmark EU (Withdrawal) Bill goes into its committee stage this week.
"The amendment that has been tabled by the Government, there is a day on which we have got to go… seems to reduce the Government's flexibility in its negotiations – it's an incoherent and thoroughly stupid amendment. It won't have my support," he said.
He said he did not believe it was too late for the country to change its direction, if people signalled that was what they wanted.
"I remain of the view that (Brexit) is an incredibly bad idea, one from which we will continue to suffer for decades to come," he said.
"I'm not in the business of trying to obstruct Brexit taking place, even though I think it's a disaster, because in the light of the referendum result I don't think it would be right for me to do so.
"(But) If the public in this country, come to the conclusion that they want something different, then I think Parliament ought to be in a position to respond," he added.
Speaking ahead of the Prime Minister's business meeting, Liberal Democrat leader Sir Vince Cable said most businesses were nervous about the Government's pursuit of a hard Brexit and needed certainty.
"Any transitional deal needs to maintain the status quo so as to provide the certainty that business needs. The Prime Minister must push for a deal that keeps us within the customs union and single market," he said.
But Tory former leader and cabinet secretary Iain Duncan Smith said Labour was shifting its position on
He told the BBC: "Staying inside the Court of Justice would actually be tantamount to staying within the European Union."
Mrs May wants to agree the broad outline of a transition deal with the European Union for after Brexit in the next few months, her spokesman said on Monday.
"We would hope we could reach an agreement on the broad outline of what an implementation period looks like within the next few months," the spokesman told reporters.