Pro-Brexit and anti-Brexit protesters demonstrate outside the Houses of Parliament in London on Tuesday. Peter Nicholls/Reuters

LONDON: Prime Minister Theresa May's Northern Irish allies and an influential group of her own MPs rejected her renegotiated Brexit terms on Tuesday ahead of a crucial parliamentary vote.

"Sufficient progress has NOT been achieved at this time," tweeted Nigel Dodds, deputy leader of the Democratic Unionist Party (DUP).

Earlier, a group of May's Conservative MPs which includes Dodds delivered a similar opinion, and urged the House of Commons to reject the deal in a vote on Tuesday night.

"In the light of our own legal analysis and others we do not recommend accepting the government's motion today," veteran Tory MP Bill Cash said on behalf of the group.

Without the support of the DUP's 10 MPs and Conservative eurosceptics, the Brexit deal looks highly unlikely to pass.

Late on Monday, May and the EU agreed fresh guarantees on the operation of a "backstop" plan in the deal, which is intended to keep open Britain's border with Ireland.

Many MPs fear it risks keeping Britain indefinitely in a customs union with the bloc.

Attorney General Geoffrey Cox said May's changes would offer Britain some protection but added that there were still circumstances in which it would be trapped in the backstop.

A DUP spokesman said: "The European Union has been intransigent.

"It is possible to reach a sensible deal which works for the United Kingdom and the European Union but it will require all sides to be reasonable and in deal making mode."

Britain is scheduled to leave the European Union on March 29.

Agence France-presse