Thirteen former cabinet colleagues wrote to British Prime Minister Theresa May to ask her not to agree Labour's demand for a post-Brexit customs union with the EU. Picture: Victoria Jones/Pool via Reuters

London - British Prime Minister Theresa May was under pressure from her own party on Tuesday to abandon a bid to find a Brexit compromise with the opposition Labour Party.

Nearly three years since the United Kingdom voted 52% to 48% to leave the European Union, there is still no agreement among British politicians about when, how or even if the divorce should take place.

It was due to have left the European Union on March 29, though May has been unable to get her divorce deal approved by parliament so she has turned to the Labour Party, led by socialist Jeremy Corbyn, in a bid to court his support.

Thirteen of May’s former cabinet colleagues as well as Graham Brady, chairman of the 1922 Committee of Conservative lawmakers, wrote to May to ask her not to agree Labour's demand for a post-Brexit customs union with the EU.

"You would have lost the loyal middle of the Conservative Party, split our party and with likely nothing to show for it," the letter said. "We urge you to think again."

"No leader can bind his or her successor so the deal would likely be at best temporary, at worst illusory," said the letter which was signed by Gavin Williamson, who was sacked as defence secretary earlier this month, and former foreign minister Boris Johnson.

The BBC's political editor said on Twitter an unidentified Downing Street source had stated that May would not sign up to a permanent customs union.

"We are trying to find a compromise on customs as interim position or stepping stone," the BBC's Laura Kuenssberg quoted the source as saying.

"Both sides agree no parliament can bind future govt + most EU trade deals have a 6-12 mnth exit clause."

Labour Party leader Corbyn said last week that May had made no big offer on Brexit and had not moved her "red lines".

May's chief Brexit negotiator, Olly Robbins, is due in Brussels to discuss changes to the political declaration on the UK's future relationship with the EU.

Robbins will explore how quickly changes could be made to the political declaration if the government and Labour can come to an agreement, the BBC said. 

Reuters