The Democratic Unionist Party is threatening to vote against the Budget – and bring down Theresa May – if it doesn’t get its way on Brexit.
The dramatic warning follows DUP unease that the Prime Minister will seek to “bounce” the party into accepting plans for more regulatory trade checks once the UK quits the EU.
The party’s 10 MPs have propped up the Tory government since May lost her Commons majority in the snap general election last year.
But HuffPost UK has been told the party has made clear to Downing Street its “blood red lines” on Brexit could not be broken.
One source told Sky News: “If Theresa May doesn’t take our concerns on board, she may not be the leader to take us through Brexit.”
May is already under huge pressure from her Tory backbenchers over her plans for a Brexit compromise that seeks to keep the UK committed to a “common rulebook” on EU trade.
With more than 40 Conservative MPs ready to vote down the May plan, it is understood that the DUP are acting “in lock-step” with the backbench European Research Group (ERG).
A spokeswoman for the DUP told HuffPost UK: “The government know our red lines on this. What’s going to happen is fundamental for future generations in Northern Ireland. They know where we stand.
“We can’t have been clearer, we don’t want to be separated from the rest of the UK.”
The Budget is due on October 29, after the next EU summit and before any special Brexit summit that is expected to finalise the deal.
Voting down the Budget would effectively be seen as a vote of no confidence in the May’s premiership as every government needs to pass its finance bills in order to govern.
The row centres on suspicions that May will make further compromises with Brussels by agreeing to new regulatory checks between Britain and Northern Ireland in an emergency plan should the UK crash out of the EU with no deal.
Some in the Cabinet are understood to be sympathetic to the DUP concerns, believing the threat can be used to force the EU to shift its own stance.
However other supporters of the PM fear the Northern Ireland party could be overplaying its hand and May will have to face them down as she did over a similar row last December.
Asked whether she could rely on DUP votes to pass her Chequers plan, a spokesman for the Prime Minister said: “The confidence and supply arrangement that we have with the DUP is a matter of record.
“The only votes that she can depend on as leader of the Conservative party are Conservative ones.”
Speaking in Brussels, EU chief negotiator Michel Barnier said that the issue of Northern Ireland was ‘key’ to an agreement. But he stressed: “Brexit was not our choice.”