The government has not proposed any changes to the PM’s Brexit deal during cross-party talks, says shadow Brexit secretary Sir Keir Starmer.
Meetings have been taking place between Tory and Labour politicians to find a proposal to put to the Commons before an emergency EU summit next week.
But Sir Keir said the government was not “countenancing any change” on the wording of the existing plan.
He said it was “disappointing”, adding: “Compromise requires change.”
The UK is currently due to leave the EU on 12 April and, as yet, no withdrawal deal has been approved by MPs.
Theresa May has written to European Council President Donald Tusk to request an extension to 30 June.
But she says if the Commons agrees a deal in time, the UK should be able to leave before European parliamentary elections on 23 May.
Sir Keir said: “We want the talks to continue and we’ve written in those terms to the government, but we do need change if we’re going to compromise.”
Prisons Minister Rory Stewart told BBC Radio 4’s PM programme that there were “tensions” but there was “quite a lot of life” left in the talks with Labour.
He said: “In truth the positions of the two parties are very, very close and where there’s goodwill it should be possible to get this done and get it done relatively quickly.”
He insisted that “of course we are prepared to compromise” on the political declaration.
BBC political editor Laura Kuenssberg said: “The sense is that the government has only offered clarifications on what might be possible from the existing documents, rather than adjusting any of their actual proposals in the two documents.”
She added that both sides agree the talks are not yet over, but there are no firm commitments for when further discussions might take place.
It comes as the prime minister is seeking a further extension to the Brexit deadline from the EU, delaying the UK’s leaving date until 30 June.
She said the UK would prepare to field candidates in European Parliamentary elections on 23 May in case no agreement is reached by then.
The BBC’s Europe editor Katya Adler has been told by a senior EU source that European Council President Donald Tusk will propose a 12-month “flexible” extension to Brexit, with the option of cutting it short if the UK Parliament ratifies a deal.
But French President Emmanuel Macron’s office said on Friday that it was “premature” to consider another delay.