Rishi Sunak’s post-Brexit trade deal for Northern Ireland faces its first hurdle next week as MPs will vote on a key part of the arrangement. The full statutory instrument relating to the Stormont brake of the Windsor framework will be published on Monday before a vote in the Commons on Wednesday, the Commons leader, Penny Mordaunt, has confirmed. The mechanism will give the UK a veto over any new EU laws applying to trade in Northern Ireland. It is not clear how the Democratic Unionist party (DUP) will vote, as the party previously said the framework did not deal with some “fundamental problems” created by existing arrangements. But the government hopes the mechanism will provide DUP MPs with enough reassurance. Hardline Brexiters will be disappointed that they will not get a chance to vote on the Windsor framework in its entirety, but No 10 has said there will be more statutory instruments to be debated in the coming weeks. “The Stormont brake we believe is at the heart of the framework. As a point of principle we are not required to bring a vote on the framework overall but obviously the prime minister made a commitment,” the prime minister’s official spokesperson said. The European Research Group of Conservative Brexiters expects its “star chamber” of experts to have reached a decision on how it intends to vote on this aspect of the deal before Wednesday. Mark Francois, the group’s chair, said: “We are still awaiting the outcome of the star chamber’s detailed legal audit of the Windsor framework, which of course includes the Stormont brake. We now hope to see this completed before next Wednesday and members of the group will no doubt pay close attention to the star chamber’s conclusions, prior to any vote.” Last month the prime minister told Tory MPs to give the DUP the “time and space” to consider his Brexit deal, as they were warned it was the best offer they would get. The arrangement has been widely welcomed by a number of Conservative MPs, including Steve Baker, who said: “I cannot see how we will get better than this … this is the deal.” Boris Johnson has been critical of the deal, claiming he would find it “very difficult” to vote for it. Sunak may have welcome relief as at the same time as the vote, Johnson will be questioned by the privileges committee as part of its inquiry into whether the former prime minister lied to MPs with his Partygate denials. Meanwhile, Tony Blair has urged the DUP to rejoin power-sharing in Stormont. Speaking to MPs on the Northern Ireland committee on Wednesday, he said: “If you want to preserve the union today, the best way of doing it is to recognise the status quo is the union, so make people comfortable with the status quo.” He praised the Windsor framework, saying it was the best practical solution to the border issues caused by Brexit. “My reason for supporting what the government has done, what this prime minister has done, on the Windsor agreement is I think it represents the most practical way forward that minimises all the theoretical objections,” he said. Announcing Commons business for the next week, Mordaunt said: “On Wednesday 22 March a debate [will take place] on a motion to approve a statutory instrument relating to the Stormont brake in the Windsor framework.” After announcing details of the framework, an optimistic Sunak said the Stormont brake – which would allow the assembly a say over EU law applied in Northern Ireland – would be a key step towards restoring the “democratic deficit”.