As part of the effort to get the deal approved by the British parliament, the EU and May set out some assurances in a choreographed exchange of letters on Monday.

The EU told May that it stood by commitments to find ways to avoid triggering the controversial "Irish backstop" in their Brexit deal and that this pledge had legal weight.

In a joint reply to questions from May, European Commission President Jean-Claude Juncker and European Council President Donald Tusk said the EU stood by its commitment to try and reach a post-Brexit trade deal by the end of next year in order to avoid using the unpopular backstop.

While stressing that nothing in their letter could be seen as changing or being inconsistent with the draft treaty agreed with May last month, they said a commitment to speedy trade deal made by EU leaders had "legal value" which committed the Union "in the most solemn manner".

However, even if the target date were not met, they wrote, Britain would have the option to extend a status-quo transition period to avoid triggering the backstop, which is meant to avoid a hard customs border for Northern Ireland.

"If the backstop were nevertheless to be triggered, it would only apply temporarily, unless and until it is superseded by a subsequent agreement that ensures that a hard border is avoided," they said.

May said the assurances might not go far enough for some lawmakers and the small Northern Irish party that props up her government said it was insufficient.

"The letter isn't legally binding," Democratic Unionist Party deputy leader Nigel Dodds told BBC radio.

May will make a statement to parliament at about 1530 GMT.

But with her deal facing opposition from all sides in the lower house of parliament, the House of Commons, the letters are unlikely to change the fundamental outcome of the vote.

Reuters