DUP MPs are prepared to vote down this month's Budget if Theresa May breaks any of the party's red lines in the Brexit talks, it has been reported.
The Northern Ireland party's 10 MPs keep May in power thanks a to a 'confidence and supply' deal agreed after the 2017 General Election.
But as time runs out for a Brexit deal, the DUP is increasingly concerned May will agree to EU demands to create a regulatory barrier between Northern Ireland and Great Britain as part of a 'backstop' provision.
BBC and Sky both reported on Wednesday afternoon the DUP were prepared to break its agreement with the Tories.
Former Brexit minister Steve Baker, who is one of those leading the charge against May's Brexit strategy, told City A.M. he was not surprised by the threat to vote down the Budget – a move that would effectively cripple the Government.
"Of course the confidence and supply agreement is at risk if the union is threatened," he said.
Historically, when a Government is unable to pass its Budget a General Election is called.
But with the Fixed Term Parliament Act needing a separate vote of no confidence to trigger an election, it could be that the Conservatives cling on to power.
One outcome could be that May is toppled by her own MPs in order for a new leader to strike a deal with the DUP and continue the Brexit negotiations with Brussels.
Baker played down the suggestion that May could be out of a job if the Budget is voted down, and said: "I don't think it necessarily follows. If she was able to find some other way to move forward in the talks she could stay as Prime Minister."
The Budget is set for October 29.