THERESA May says Brexit could be delayed if there are “exceptional circumstances”.

The admission from the Tory leader came as the Government finally backed down over plans to fix the Brexit date in law.

The Government had initially tabled an amendment to the EU (Withdrawal) Bill which, if passed, would have made the UK coming out of Europe on 11pm on March 29 2019 enshrined on the statue books.

Many in the Prime Minister’s own party rubbished the proposals, saying it was unnecessary and would hamper the Government’s ability to negotiate with Brussels.

Facing near certain defeat, the Tory leader opted to compromise.

She gave her backing to an amendment from Sir Oliver Letwin which would include the date in legislation but also give ministers the flexibility to change.

The problem for May is that many of the more obsessive Brexiteers in her party are worried that her heart isn’t in it, and that this could be used to delay Brexit indefinitely.

During Prime Minister’s Questions, senior Tory Julian Lewis asked for assurances that the amendment to “change the date of our leaving the EU will be invoked only if at all under extremely exceptional circumstances and only for a very short period”.

May said she was happy to give that “reassurance”, adding: “We’re very clear we will be leaving the EU on March 29 2019 at 11pm.

“The Bill that is going though does not determine that the UK leaves the EU, that’s part of the Article 50 process and it’s a matter of international law and it’s important, I think, that we have the same position legally as the European Union and that’s why we have accepted the amendment”.

Though she avoided defeat last night, there was confusion over whether she would honour her defeat in the Commons from last week.

Then, MPs teamed up to force the government into committing to a meaningful vote on Brexit.

But yesterday as May was in front of the Commons’ Liaison Committee made up of Select Committee chairs there was some disagreement on what exactly that vote would be on.

Five times, the Prime Minister was asked to guarantee the vote would be on a Bill, allowing it to be amended, or for her to be sent back to Brussels to seek better terms.

May did not give that guarantee.

Asked, by Labour MP Yvette Cooper, to confirm there would be a “vote on a statute” – on legislation – the Tory leader said only that there would be an “opportunity to vote on the deal”.

In the Commons, former Tory Attorney General Dominic Grieve, who headed up the rebel alliance last week, warned ministers that “kicking hornets’ nests is not a very good idea”.

That came as Brussels indicated they were amenable to a transition period after the UK leaves the EU, but only if it ends by December 2020.

Agreement would be dependent on Britain signing up to the single market, and taking on all the rules and regulations without having any say in what they are.

Though the transition will help avoid a cliff edge, it will be hard to swallow for some of May’s more zealous Brexiteers.

Perhaps most difficult of all will be the need for rulings of the European Court of Justice to continue to apply.

The Prime Minister had initially suggested the transition period could last two years from Brexit day.

But yesterday, Brussels said it would need to be four months shorter.

“The transition period needs to be clearly defined and precisely limited in time,” the EU says.

“The commission recommends that it should not last beyond 31 December 2020.”

May insisted to MPs that the transition could possibly be longer, and that it would be a matter for negotiation.

The Prime Minister told the committee the December date represented a “neatness” for Brussels, but she was sure negotiators would be open to extending the transition window.

The Tory leader also told the committee she believed the entire agreement about the UK’s future relationship with Europe could be negotiated by Brexit day: “That is what we are working to and that is what I believe we can do.”

Meanwhile, the UK Government claimed they were closer to securing the consent of the administrations in Cardiff and Edinburgh for the repeal bill. Damian Green told a committee of Lords it was “not in the interests of people” for “there not to be a working statute book.”