The Democratic Unionist party (DUP) is to vote against the government in this week’s first parliamentary vote on the new Windsor framework Brexit deal. Party officers met on Monday and made a unanimous decision to reject Rishi Sunak’s revised plan for post-Brexit trade arrangements in Northern Ireland, the DUP leader, Sir Jeffrey Donaldson, said in a statement. “It does not deal with some of the fundamental problems at the heart of our current difficulties. There remain key areas of concern which require further clarification, reworking and change as well as seeing further legal text,” Donaldson said. The decision will embolden the European Research Group (ERG), which represents Conservative Brexit hardliners, to also vote against the government. But Labour’s support guarantees the motion, which overhauls the Northern Ireland protocol, will pass. Donaldson offered an olive branch to Downing Street by saying the DUP would continue to work with the government “on all the outstanding issues relating to the Windsor framework package” to try to restore power sharing in Northern Ireland. The vote on a statutory instrument to implement the “Stormont brake” is being viewed as a proxy for MPs to have their say on the whole deal. The proposed mechanism would give the Stormont assembly a greater say over the implementation of EU law in the region and let the UK government ultimately veto any new EU laws applying to trade in Northern Ireland. Donaldson has avoided giving a definite verdict on the deal – which was sealed by Sunak and the European Commission president, Ursula von der Leyen, on 27 February – by saying the party needed time to study the fine print. Monday’s statement stopped short of an outright rejection but repudiated a centrepiece of Sunak’s deal. Donaldson said: “It remains the case that the ‘brake’ is not designed for, and therefore cannot apply, to the EU law which is already in place and for which no consent has been given for its application. “Whilst representing real progress, the ‘brake’ does not deal with the fundamental issue which is the imposition of EU law by the protocol.” Speaking later to the BBC, Donaldson was blunter: “This doesn’t work for Northern Ireland. We need to get this right.” The statements all but extinguished any lingering hope that the Stormont institutions could be revived in time for the 25th anniversary of the Good Friday agreement on 10 April. The US president, Joe Biden, and Bill and Hillary Clinton are among the dignitaries who are to visit Northern Ireland next month to mark the agreement. There has been speculation Donaldson may allow the institutions to revive while maintaining formal opposition to the Windsor framework. Donaldson has come under intense pressure from London, Dublin and Washington to drop the DUP’s boycott of the Stormont assembly and executive, which has paralysed devolved government since last year. However, factions in the DUP and hardline unionist rivals outside the party favour keeping the boycott. The DUP MP Ian Paisley has been outspoken in saying Sunak’s deal failed to address the party’s seven tests. Sammy Wilson, another DUP MP, went further by accusing Chris Heaton-Harris, the Northern Ireland secretary, of making wild and misleading claims about the Stormont brake. Details of the legislation published on Monday showed the UK government could in certain circumstances ignore the wishes of Stormont assembly members who applied the brake, rendering the safeguard “useless”, Wilson said. “The wildest assertions are being laid bare,” he added. Concerns have grown among Conservative whips that some Tory MPs could also vote against or abstain. The former deputy Commons leader Peter Bone 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.