Image copyright EPA/Getty Images Image caption DUP could scupper May's Brexit plans

Northern Ireland's Democratic Unionist Party is planning to vote against the Budget if Theresa May breaches the party's Brexit red lines, Newsnight has learnt.

The party would end its parliamentary support for the prime minister if she agrees a deal at next week's EU summit that led to additional checks between Northern Ireland and the rest of the UK.

The loss of the DUP, which won ten seats at the last election, would raise severe doubts about the government's ability to pass the Budget which is due to be delivered on 29 October.

Losing a budget vote has traditionally been seen as a withdrawal of confidence in the government. One former Tory cabinet minister told Newsnight: "The DUP should be putting the fear of God into Downing St."

The DUP is growing alarmed because it fears Downing St is edging towards a deal with the EU that may lead to additional regulatory checks on goods travelling between Great Britain and Northern Ireland.

'No policy change'

Michel Barnier, the EU's chief negotiator, has called for such checks to avoid creating a hard border between Northern Ireland and the Irish Republic. This would be achieved by aligning Northern Ireland with the rules of the single market.

The prime minister has strongly rejected the Barnier plan. But senior DUP sources fear that Downing St may agree to some form of regulatory checks.

One DUP source told Newsnight: "If we are not happy with what happens next week [in Brussels] we won't be bounced into anything. If she doesn't take our concerns on board, we will take the view that Theresa May is not the leader to take us through to a safe Brexit."

The DUP was concerned after Mr Barnier reportedly told the party in Brussels this week that Great Britain is entitled to sign a traditional free trade deal with the EU. But Northern Ireland would have to be separate and subject to the rules of the single market to avoid a hard border on the island of Ireland.

The prime minister has consistently said that no UK prime minister would agree to hiving off Northern Ireland from the rest of the UK. Mel Stride, the financial secretary to the Treasury, insisted that there has been no change in government policy.

'Nuclear' red lines

Asked about the BBC report that the DUP might vote down the budget, he told Radio 4's The World at One: "I'm not going to speculate on something that just isn't going to happen. We are extremely clear that there will be no border down the Irish Sea."

But the DUP fears that a two-part Brexit deal is emerging that would breach its "nuclear" red line. That is its code for pulling the plug on Theresa May even if that led to Jeremy Corbyn becoming prime minister.

The DUP believe the government's possible Brexit deal would lead to:

A temporary arrangement in which the UK as a whole would be in some form of customs union with the EU, though it would have a different name. The DUP would regard this as a betrayal of Brexit, but not a breach of their "nuclear" red line because Northern Ireland would be treated the same as the rest of the UK.

Some form of regulatory checks on goods travelling from Great Britain to Northern Ireland. That would be a breach of the "nuclear" red line because Northern Ireland would be treated differently to the rest of the UK with additional checks.

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