British leader Theresa May arrives for a church service near her Maidenhead constituency on Sunday. Photo: AFP

UK Prime Minister Theresa May warned on Sunday that if parliament rejects her Brexit deal, it could leave Britain in the European Union and put the opposition Labour Party in power.

May said Britain “would truly be in uncharted waters” if the text agreed after nearly two years of painstaking negotiations is voted down less than four months before the March 29 Brexit date.

“It would mean grave uncertainty for the nation with a very real risk of no Brexit,” she told the Mail on Sunday.

“We have a leader of the opposition who thinks of nothing but attempting to bring about a general election… I believe Jeremy Corbyn getting his hands on power is a risk we cannot afford to take.”

The beleaguered leader’s message came with her government fearing a heavy defeat in Tuesday’s parliamentary vote on the draft withdrawal agreement she signed with Brussels last month.

Media reports said May is under pressure from her cabinet to delay the vote and fly to Brussels to secure more concessions ahead of a planned summit with the other 27 EU leaders on Thursday and Friday.

However, Brexit Secretary Stephen Barclay told the BBC: “The vote is going ahead.”

– with reporting by Agence France-Presse