Brexit could be delayed until after March 2019, Theresa May has conceded – but insisted it would only be “in exceptional circumstances for the shortest possible time”.

The Prime Minister came under pressure in the Commons after her climbdown over putting the exact date and time of departure in domestic law.

A last-gasp compromise – to be confirmed tonight – will give MPs the power to over-ride that commitment and put back the exit date if the Brexit negotiations run into trouble.

Two Brexit-backing Tories made clear their unease, leading Ms May to acknowledge – for the first time – that March 29, 2019 will not be set in stone as the exit day.

She confirmed the Government would support the amendment, allowing Brexit day to be delayed, but told MPs she would only “use this power in exceptional circumstances, for the shortest possible time”.

And she tried to head off opposition on the Tory benches by insisting: “We’re very clear we will be leaving the EU on March 29 2019 at 11pm.”

Julian Lewis, a former shadow minister, was the first Tory backbencher to raise concerns that amendment 400 to the EU Withdrawal Bill would allow MPs to “change the date of our leaving the EU”.

Another prominent Eurosceptic, John Baron, echoed that concern, seeking a reassurance from the Prime Minister that any delay would be for only a few “weeks or two months”.

In reply, Ms May again insisted she anticipated any holdup would be for the “shortest possible time”.

The Prime Minister was facing the prospect of a second defeat on her flagship Brexit legislation – just one week after her humiliation over a “meaningful vote” on the exit deal.

But the crucial amendment, tabled after pro-EU Tories warned they had the numbers to win the vote, is seen as way for her to save face.

The exact date and time – 11pm, on March 29, 2019 – will still be on the face of the Bill, as Ms May pledged in a blaze of publicity last month.

However, MPs will enjoy the “power to amend the definition of ‘exit day’” at a later date, providing the potential for the UK to ask the EU to extend the Article 50 negotiations if necessary.

Dominic Grieve, the former Attorney General who led last week’s revolt, has said that the new amendment left him “fairly satisfied”.

It has the support of other leading Brexit supporters, including Jacob Rees-Mogg and Sir William Cash, as well as Remain supporters George Freeman and Nick Boles.

Ms May’s comments came as the European Union said it wanted the planned Brexit transition period to end no later than 31 December 2020.

“The transitional arrangements should apply as from the date of entry into force of the Withdrawal Agreement and should not last beyond 31 December 2020,” the European Commission’s latest negotiating directives said.

The deadline coincides with the end of the EU’s seven-year budget period – but appears earlier than the Prime Minister’s asked for transition of “around two years”.