British foreign minister Jeremy Hunt warned on Sunday that Brexit is in peril if British Prime Minister Theresa May's Brexit deal is rejected.

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He said that there was momentum behind a move to stop Britain leaving the European Union and Brexit could be reversed unless lawmakers backed the government's exit deal on Tuesday.

"We have an opportunity now to leave on March 29 or shortly thereafter and it's important we grasp that opportunity because there is wind in the sails of people trying to stop Brexit," Hunt told the BBC.

"If you want to stop Brexit you only need to do three things: kill this deal, get an extension, and then have a second referendum. Within three weeks those people could have two of those three things ... and quite possibly the third one could be on the way."

Facing the deadlock, Health Minister Matt Hancock also warned that the country will face "total uncertainty" if lawmakers reject May's Brexit deal, adding that he did not think a defeat was inevitable.

"In the event of this vote on Tuesday not going through, nobody knows what would happen. In everything that followed there would be total uncertainty - that would be the only certainty," Hancock told Sky News.

Meanwhile, the leaders of two major Eurosceptic factions in parliament said Sunday that May's Brexit deal will face a heavy defeat in parliament on Tuesday because she has so far securedno major changes from the European Union.

"An unchanged withdrawal agreement will be defeated firmly by a sizeable proportion of Conservatives and the Democratic Unionist Party (DUP) if it is again presented to the Commons," deputy DUP leader Nigel Dodds and Conservative party's leading Eurosceptic Steve Baker wrote in the Sunday Telegraph.