The contest to succeed Theresa May as Britain's next leader heated up on Monday, with candidates lining up to define themselves against the "bluff and bluster" of Brexit-backing frontrunner Boris Johnson.

Conservative MP Boris Johnson leaves a house in London on June 7, 2019. [Photo: AFP]

Ten Conservative MPs gathered enough support to qualify by Monday's deadline for the race to replace May, who quit as the party's leader on Friday over her failure to take Britain out of the European Union on time.

She remains premier until her replacement is chosen, likely in late July, and whoever it is will automatically enter Downing Street.

The competition is dominated by Britain's looming EU exit on October 31, with leading Brexiteer Johnson, the former foreign secretary, among those talking tough on the need to renegotiate the divorce terms or leave without a deal.

The bombastic former London mayor has been keeping a low public profile but is campaigning hard behind the scenes, and his rivals tacitly acknowledged he is the one to beat.

Environment Secretary Michael Gove, who is battling a scandal over his admission that he took cocaine, jeered at Johnson for shunning the limelight.

"We need someone who has been tested in the heat of battle.. not someone who will hide in their bunker," said Gove, who also criticized Johnson directly for his newly-announced proposal to cut taxes for the rich.