DUP ready to vote down budget and potentially topple Theresa May if she compromises further on Brexit

Budget votes are viewed as an issue of confidence in a government – meaning defeat would trigger calls for a general election

The Democratic Unionist Party says it is ready to block the budget and potentially topple Theresa May if she compromises further on Brexit.

The Northern Ireland party – which is propping up the Conservatives in power – dramatically threatened to pull its support if its “red lines” over the Irish border are crossed.

The move comes as the UK and the EU edge towards an agreement that would allow regulatory checks between Britain and Northern Ireland, enraging the DUP.

Arlene Foster has described its red lines as “blood red”, prompting suggestions that the party could join with opposition parties to vote down the budget, on 29 October.

A DUP source told The Independent: “The government is very well aware of what our red lines are and it knows the importance we place on them.”

Asked if that extended to defeating the government in the crucial budget vote, the source replied: “I don’t think that’s a misplaced conclusion.”

Budget votes are viewed as an issue of confidence in a government – meaning defeat could fatally undermine the prime minister’s position and would trigger calls for a general election.

But Downing Street later insisted that losing a vote on the budget would not be confidence issue – and therefore a resigning matter – because the fixed-term parliament act had changed the rules.

The DUP had already threatened to vote against any Brexit deal if it created any new regulatory checks on goods crossing the Irish Sea, even if they take place away from ports.

But extending that threat to the budget is even more serious, because it would rip apart the terms of the “confidence and supply” agreement with the Tories – the “supply” part being money, through a budget.

Until now, Conservatives have been confident that the DUP’s distrust of Jeremy Corbyn would prevent it doing anything that brought Labour closer to power.

Earlier, Ms Foster pointed out that the confidence and supply arrangement was “party to party”, rather than with Ms May herself – suggesting the DUP might be willing to bring her down.

Under the fixed-term parliaments act, losing a confidence vote no longer automatically triggers an election, because it creates a breathing space for a new government – under a new prime minister.

Ms May’s spokesman dismissed the idea of the budget vote bringing down the government, saying: “The fixed-term parliament act sets out the circumstances for a confidence vote.”

Mel Stride, a Treasury minister, tried to calm DUP nerves, insisting the prime minister had been “crystal clear on this issue of no border down the Irish Sea”.

“I am extremely confident from all that I know, that I have seen and all the discussions that I have had, that there will be a very, very firm position taken on this,” he told the BBC.

“The prime minister made it very clear no UK prime minister is ever going to put him or herself in the position where they start to unpick the economic and sovereign integrity of the UK.”

Ms May will meet EU leaders in Brussels next week to try to seal the deal on the Irish border – with plans that have yet to be shown to either MPs or the cabinet.

They would also see the entire UK remain in the EU’s customs territory until technology can prevent a hard border, raising Brexiter fears that the arrangement will become permanent.

Earlier, at prime minister’s questions, Ms May appealed to Labour MPs to put the “national interest” ahead of loyalty to Mr Corbyn when it comes to a vote on her Brexit plans.

The plea appeared to be aimed at her own MPs, after reports that up to 80 hardline Eurosceptics are ready to defy her and vote it down.

Labour dismissed claims that up to 30 of its MPs were prepared to back the government. Mr Corbyn’s spokesman said there was no evidence for it.