Boris Johnson has urged supporters of Nigel Farage's Brexit Party to vote for the Tories in Thursday's crunch Brecon and Radnorshire by-election.
The Prime Minister faces having his Tory-DUP majority cut to just one if the Liberal Democrats take the seat.
Mr Johnson, who is expected to visit the seat on Tuesday, said: "A vote for any party other than Conservatives pushes the Liberal Democrats one step closer to cancelling the referendum result.
"The Brexit Party cannot deliver Brexit, only the Conservatives can."
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