May's possible Brexit deal could be backed by 30-40 Labour MPs

Jacob Rees-Mogg

Jacob Rees-Mogg

But the unionists are warning of a revolt if Mrs May agrees to the terms the European Union are calling for.

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

Some Brexiteers say those proposals would ensure the EU kept control over swathes of the British economy and thus run counter to the spirit of her manifesto pledge to leave the EU Customs Union and the Single Market.

There are usually checks on goods coming into the EU's customs territory from outside countries, but the bloc has agreed not to carry out such checks between Northern Ireland and the Republic of Ireland after Brexit in order to safeguard the peace process.

In Northern Ireland, 86 per cent of Leave voters believe Scottish independence would be acceptable if meant securing the UK's withdrawal from the EU.

However he said other products could be dealt with by scanning barcodes on lorries or shipping containers.

DUP MP Sammy Wilson has said his party "will not be bullied" into propping up Theresa May's Government if she gives in to the "outrageous, capricious and extortionate" demands of Brussels on Northern Ireland.

And we are still open - we are still open - to the idea of having a customs union with the UK.

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 Theresa May doesn't take our concerns on board, we will take the view that she is not the leader to take us through to a safe Brexit".

Treasury Minister Mel Stride responded: "The Prime Minister's position has always been crystal clear on this issue of no border down the Irish Sea - we have said it many, many times".

Budget motions are generally seen as a vote of confidence, which means the ominous threat from the DUP could cast doubt on Mrs May's future.

The Democratic Unionist Party is prepared to vote against the Budget if their red lines for an European Union deal are ignored next week, Sky News understands.

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.

In a speech in Brussels, Barnier reiterated his rejection of the counter-proposals hammered out by the cabinet at Chequers, which Theresa May insists is the only deal that respects both the referendum result and the constitutional integrity of the United Kingdom by ensuring "frictionless" trade and no hard border.