Britain's Prime Minister Theresa May | Ben Stansall/AFP via Getty Images Theresa May concedes ECJ rule during Brexit transition Prime minister also says British government is preparing in case UK leaves EU without a deal.

LONDON — Another week of Brexit negotiations, another incremental but consequential shift in the U.K.’s position.

Theresa May said Monday that Britain would continue to be bound by European Court of Justice rulings during any future transition period and refused to rule out accepting new Brussels regulations brought in during that time.

Responding to backbench MP Jacob Rees-Mogg during Brexit questions in the House of Commons, the prime minister said Britain wanted an orderly withdrawal from the EU and "that may mean we will start off with the ECJ governing the rules that we are part of."

May said it was "highly unlikely" that new rules would be brought in during an implementation period that the U.K. had not already agreed to before leaving. However, the prime minister refused to rule out the prospect of accepting new rules fast-tracked onto the EU statute book during the prospective two-year transition.

Her concession on the ECJ risks opening a rift with supporters of a hard Brexit on her backbenches, and a further destabilization of her government, which has been rocked by Cabinet in-fighting and discontent among MPs, which culminated in a coup attempt, albeit a small-scale one, last week.

Brexiteer MPs were quick out of the blocks last night to voice their concern at the prime minister’s language. Bill Cash, a veteran Euroskeptic, told Sky News there were “questions that have yet to be answered,” adding that “a lot of people” would be “looking very, very carefully at the words” May had used.

Rees-Mogg, the current darling of those Tory members who support a quick Brexit, told the same broadcaster that May had “to some extent” gone “further than I would have wished in offering a generous and comprehensive deal to the European Union.”

During the Commons debate Monday afternoon, May also told MPs that she wanted a free-trade deal with the European Union signed by March 2019 but warned she had already begun preparing for a no-deal scenario in case that was not possible.

Following a question from the former Conservative Party leader Iain Duncan Smith, May said there was an "increasing interest" among European leaders to move onto trade talks to enable a final deal to be agreed within the next 18 months.

"We want [the free-trade deal] negotiated by March 2019 so the U.K. will come out of the European Union and we will know what that new relationship will be," she said.

Privately, U.K. government negotiators have conceded that the EU is unlikely to declare “sufficient progress” at next week’s European Council meeting. December’s summit is now viewed as the pivotal moment, according to one senior U.K. negotiator. However, optimism is growing that the EU will soon agree to commence talks about trade and a transition period.

However, May warned that it was "prudent" to prepare for a failure in the negotiations and revealed that the two new government white papers published Monday, which set out proposed new legislation on trade and customs after Brexit, contained steps to reduce disruption in the event of a "no deal."

“While I believe it is profoundly in all our interests for the negotiations to succeed, it is also our responsibility as a government to prepare for every eventuality, so that is exactly what we are doing," she said.

“These white papers support that work, including setting out steps to minimize disruption for businesses and travelers.”

Responding, Labour Party leader Jeremy Corbyn accused the prime minister of wasting 16 months since the referendum with no progress being made. He added: "The Florence speech demonstrated the scale of the mess the government is making of these negotiations."