The Democratic Unionist Party said on Wednesday it would withdraw support from the government's budget plan if Theresa May breaks their “red line” on the Irish border issue.
Party leader Arlene Foster said earlier this week that a deal that would have Northern Ireland treated separately to the rest of the UK by the EU post-Brexit would be unacceptable to the DUP.
“We cannot have the single market of the United Kingdom interfered with in that way and that is the message we will be giving to Michel Barnier today. There cannot be any regulatory barriers between ourselves and the rest of the United Kingdom,” she said.
“I want to see a deal that works for everyone and I think that is eminently possible if the political will is there to make it happen,” Foster told BBC Radio Ulster on Tuesday.
Mel Stride, the financial secretary to the Treasury, told the BBC the government had a “very strong position” that there would be “no border down the Irish Sea”.
He believes a deal is within grasp and would be keeping the DUP “on board”.
“There is no difference between our position on the substantive issue,” he added.
Budget votes are traditionally seen as confidence votes, and losing one could lead to a no-confidence vote and trigger a general election. The DUP suggested that, if the party were to vote down a budget, it would be to bring down Theresa May and get her replaced by an alternative Tory, not call on new elections.
Theresa May relies on DUP support in the budget vote on 29 October since she does not hold a majority in the House of Commons.