The DUP is preparing to vote against Rishi Sunak’s Brexit deal with the EU in Wednesday’s crunch House of Commons vote, The Telegraph understands.
The decision by the Unionist party is likely to embolden Tory Eurosceptics who are also gearing up to oppose the Prime Minister’s deal.
On Wednesday, MPs will debate the “Stormont Brake” - a key measure in the Windsor Framework that Mr Sunak struck with the EU to improve the functioning of the controversial Northern Ireland Protocol.
Under the brake, members of the Northern Ireland Assembly would be able to block EU regulations, but only in specific circumstances and with the EU able to take remedial action in response.
Wednesday’s vote will be on a statutory instrument to implement the mechanism, but the Government has said it will be interpreted as the defining verdict on the overall agreement.
Ministers had harboured hopes that the DUP would not actively oppose the deal, paving the way for the party to return to the Stormont Assembly, which it has been boycotting for more than a year in protest at the protocol.
However, a senior DUP source told The Telegraph: "I would fully expect that we will be voting against that statutory instrument.”
The source said there were “fundamental problems still outstanding” with the Windsor Framework, pointing to comments from the DUP’s leader, Sir Jeffrey Donaldson, who last week said there were “key areas of concern” requiring “further clarification, re-working and change”.
The source also criticised Mr Sunak’s choice of parliamentary tactics.
They said: “You can’t take one part of [the Windsor Agreement] and leave aside all the rest, especially at the same time as they’re briefing out that this would be taken as a signal on the overall agreement, which of course it would be by the Government.
“The whole tactics do reek of party political games.
“This vote has been brought forward without the necessary information and texts and details being provided. It’s just an outrageous attempt to try to force this through without proper scrutiny or debate or vote.”
Disquiet at the perceived refusal to share information is shared by the European Research Group of Tory MPs.
The Eurosceptic group is yet to announce its position on the Windsor Framework, but it is expected that a number of its members will vote against the Government or abstain.
Mr Sunak has insisted that the amount of EU law applicable to Northern Ireland has been reduced to 3 per cent, scrapping 1,700 pages of law. But several members of the ERG have tabled parliamentary questions about which laws make up the 3 per cent and say they have not received satisfactory responses.
"We want a list of the laws that remain and a full list of the 1,700 pages of law that has been swept away," said one MP. "The only answers we have had have been nonsensical."
Another complained that they felt they were being "rushed" to make a decision by Wednesday's vote, adding: "The Government owes us a lot of information. It won’t even tell us which EU laws apply in Northern Ireland. The DUP are livid, it doesn't meet their tests. They are cross for the same reasons that we are."
Several members of the ERG said they had not yet made up their mind on how they will vote, adding that they would wait for the findings of their “Star Chamber” of lawyers, as well as the formal verdict of the DUP, both of which are expected to be published in the next couple of days.
Close attention is also being paid to Boris Johnson. Earlier this month the former prime minister declared that he would find it “very difficult” to vote for the deal.
Labour have said they will back the agreement, meaning it is almost certain to be passed. However, a revolt by the DUP and Brexiteer Tories could undermine the credibility of the deal and strike a blow to Mr Sunak’s authority.
A Government spokesman said: “On Wednesday, Parliament will vote on the statutory instrument for the Stormont Brake – the most significant part of the Windsor Framework.
“The brake puts power back into the hands of Stormont and Westminster, addresses the democratic deficit and restores the balance of the Belfast Good Friday Agreement.”
Downing Street has been scrambling to shore up DUP support ahead of Wednesday's vote. Unionists are understood to have a number of concerns, including that nationalist support would be required to trigger the Stormont brake.
They were also said to be anxious that even in the event the brake was triggered that the UK government would be able to veto it or the EU could prevent it from being used.
Officials at Number 10 have insisted that no vote would be needed in the Northern Ireland assembly to trigger the brake, which can be enacted with the support of 30 unionists from two parties.
They have also said it would be unlawful for the UK government to veto the brake and that while the EU can ask for more information, it cannot stop the brake from being used.
Downing Street has also highlighted that the statutory instrument will have the same status as the legislation implementing the Belfast Good Friday Agreement, and will codify the break into the Northern Ireland Act 1998.
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