Rishi Sunak faces a reckoning from hardline Brexiters on his backbenches who are prepared to follow the Democratic Unionist party in voting against a key element of his “Windsor framework” that overhauls the Northern Ireland protocol. Anger is rising among those in the European Research Group at ministers using a vote on a statutory instrument to implement the “Stormont brake” on Wednesday, as a proxy for MPs to have their say on the whole deal. After weeks of scrutinising the detail of the agreement, which was sealed by Sunak and the EU Commission president, Ursula von der Leyen, on 27 February, the DUP have been signalling their discontent more strongly. Jeffrey Donaldson, the DUP leader, said on a visit to Washington DC a few days ago: “It is my current assessment that there remain key areas of concern which require further clarification, reworking and change as well as seeing further legal text.” Concerns have since grown among Conservative whips that some Tory MPs could also vote against or abstain on Wednesday – even though Labour’s support guarantees the motion will pass. The vote will be on a statutory instrument that will implement a mechanism designed to appease concerns about the jurisdiction of the EU law. Under the new Windsor framework, the UK government can ultimately veto any new EU laws applying to trade in Northern Ireland. After a senior DUP source told the Sunday Telegraph the party was likely to vote against the government on Wednesday, several Tory MPs privately suggested they might follow suit. Peter Bone, the former deputy Commons leader, also said he was “very unhappy” about the statutory instrument vote being treated as MPs’ one chance to have a say on the Windsor framework. “I’ve not been given a reasonable explanation as to why it’s being done that way,” he said. “The government is starting from a point of rubbing me up the wrong way on this. The whole thing about the Windsor framework is everyone agrees it’s an improvement, but this isn’t going to be a temporary measure – this is permanent. “It’s a bit like a budget. When it was announced, it all sounds good – but then you have to go away and look at the small print to see how good it really is. I haven’t made my mind up yet.” The ERG is understood to be planning to hold a press conference on Tuesday, to announce the verdict of its “star chamber” of legal advisers who have been examining the detail and implications of the Windsor framework. Oliver Dowden, the Cabinet Office minister, said on Sunday he was “confident” the vote would pass voiced hopes the DUP would “come on board”. Labour vowed to support any deal to overhaul the protocol, so Sunak would not need to be in hock to his party in order to pass a deal. Lisa Nandy, the shadow levelling up secretary, told Sky: “There’s no question that this is something that is now urgent. It’s incredibly important, and trying to remove some of that friction, some of those barriers on the island of Ireland has long been our priority.”