Theresa May has acknowledged "exceptional circumstances" might result in a short Brexit delay.

The Prime Minister confirmed the Government has backed down over plans to fix in law EU withdrawal at 11pm on March 29 2019, a measure which also removed the power of ministers to choose an exit day via regulations.

Instead, the Government has offered its official support to amendments designed to ease Tory fears and avoid a second bruising Commons defeat over the European Union (Withdrawal) Bill.

Amendments 399 and 400 enable the date to be included in legislation while also giving ministers the flexibility to change it if needed.

Mrs May said the Government remains committed to leaving on March 29 2019 at 11pm although the powers to change the date would be used only in "exceptional circumstances for the shortest possible time".

The new amendments were first tabled by senior Tory MPs including Sir Oliver Letwin and Bernard Jenkin, and are supported by some of those who rebelled last week to ensure Parliament has a meaningful vote on any Brexit deal.

Speaking at Prime Minister's Questions, senior Tory Julian Lewis asked for assurances that the provisions set out in amendment 400 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".

Mrs 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 through 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 from (Sir Oliver).

"But I can assure (Mr Lewis) and the House we would only use this power in exceptional circumstances for the shortest possible time, and an affirmative motion will be brought to the House."

The Bill is currently on its eighth and final day at committee stage, which will see MPs finish their line-by-line assessment of the measures which transfer European law into British law.

Conservative former attorney general Dominic Grieve began with a criticism of schedule five of the Bill, which requires the Government to publish copies of EU law to be retained in UK law.

Under the same schedule ministers are given the power to make exceptions if they are satisfied that it "has not" or "will not" become UK law on exit day.

Mr Grieve raised concern there did not appear to be "any limitations" on that power and there "doesn't appear to be any guidance" on when such action might be appropriate.

He later suggested a "bit of focus" is needed over schedule five, and advised ministers to reflect over Christmas.

Mr Grieve said: "It'd be quite nice for the Christmas period to be taken in some quiet reflection on just how wide these powers and whether indeed, yet again, the Government might not on reflection be able to circumscribe them a little bit so they appear to be slightly less stark in terms of the power grab that's implied."

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Brexit minister Steve Baker confirmed there would be an amendment to the Bill at a later stage so MPs would have to approve a change in the exit day.

Sir Oliver said: "I hope that the minister will confirm this from the Despatch Box that the Government has said throughout this discussion that, and I think it has to be at report because of the way the Bill proceedings work, that it will bring forward an amendment to make sure also that the statutory instrument which might be triggered under amendment 400 would be by affirmative resolution."

Intervening, Mr Baker said: "I am happy to tell the House that is the case as I shall confirm later."

Sir Oliver said he was "delighted" by the confirmation, saying it should be something that Parliament can do rather than ministers on their own.

Mr Grieve thanked Mr Baker for finding a way of "resolving" the issue, and said: "The oddity of the original 381 amendment was it imposed a rather serious obstacle if for any reason there had been an agreement for the Article 50 period to end earlier."

The announcement came after Sir Oliver said there was "no question of the Government ever having asserted that they couldn't change the Article 50 date, they have always said and always known that it is possible to change".

But Jacob Rees-Mogg said the two-year timeframe in Article 50 was a "deadline" rather than the "point at which we necessarily leave, it's the point at which we leave in the event that no deal has been reached beforehand".

"It is perfectly possible should the negotiations go well for an earlier date to be agreed."

Sir Oliver said Mr Rees-Mogg was right, but that if the opposite occurred, and negotiations take longer than anticipated, an agreement could be reached to extend the date.

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Conservative former minister George Freeman welcomed the Brexit date compromise, adding: "It feels to me at this Christmas moment that not since the soldiers met on the fields of no man's land singing Silent Night has peace broken out at such an opportune moment."

Mr Grieve replied: "I am filled with the Christmas spirit of my honourable friend and very much wish to see that be present not only at Christmas but, if I may say, carried through into the new year and indeed for many years to come."

Shadow Brexit minister Paul Blomfield earlier said the new Government amendments showed that "in the face of defeat, they have caught up with us on the need for flexibility on exit days for different purposes".

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Brexit Minister Steve Baker said in accepting Sir Oliver's amendment the Government was "listening to the concerns of the House".

Speaking on Sir Oliver's amendment he said: "The amendments provide the Government with the technical ability to amend this date, but only if the UK and EU unanimously decide to change that exit day."

Later in the debate Mr Baker clashed with senior Tory Ken Clarke who had tabled an amendment which would enshrine the objectives set out by the Prime Minister in her Florence in law.

Mr Baker described the Father of the House's amendment as "novel" and said it would "constrain" the Government in negotiations.

Mr Clarke said: "In what way does it restrain the Government's position to put it its own policy on the face of the Bill?"

He added: "I cannot see what harm is therefore done by giving the approval of the whole house to the Government's stated objectives in the Bill."

Mr Baker said: "It would be a constitutional innovation to begin putting out statement of policy in negotiations on the face of legislation, that is a good reason to accept it."

Mr Clarke later told MPs: "I hope we won't have the extraordinary spectacle of fear of right wing Eurosceptics going to such lengths that the Government's putting a three-line whip on its ministers and all its backbenchers to cast a vote against the Florence speech so that some room is left for them to be able to negotiate further with either the Environment minister or the Foreign Secretary or whoever it is wants to reopen it again."