The government has promised to underline Northern Ireland’s place in the UK with legal “assurances” in an effort to convince unionists to back Rishi Sunak’s Brexit deal. Chris Heaton-Harris, the Northern Ireland secretary, said on Friday that the government would introduce legislation to allay Democratic Unionist party (DUP) concerns about the Windsor framework. Northern Ireland’s place in the UK was secure, he told the BBC. “So we’ll be looking to bring forward amendments to the Northern Ireland Act of 1998 to provide further assurances on that matter. Reassurances in law that Northern Ireland remains an integral part of the United Kingdom, and it’s the government saying that in primary legislation which is what people are asking for.” The offer was an attempt to quell mounting pressure on Sir Jeffrey Donaldson, the DUP leader, to reject the revised Brexit deal for Northern Ireland that was announced on Monday. Donaldson has said the party would take its time – possibly weeks – to study the deal before giving a verdict. However, some high-profile DUP figures and hardline unionists outside the party have already signalled opposition, saying the deal eroded Northern Ireland’s place in the union. Heaton-Harris said the “groundbreaking” agreement was “very good” for Northern Ireland and would not be renegotiated. “This is the deal we have done with the EU and this is what is on the table.” He said the “Stormont brake” that would let Northern Ireland politicians under certain circumstances block EU laws would be codified in the coming days. Heaton-Harris also said the deal would deliver prosperity that in turn would cement public support for Northern Ireland remaining in the UK. The minister made similar points in a statement on Thursday evening, underlining the government’s eagerness to get the DUP onboard. The potential lifeline to Donaldson elicited a rebuke from Colin Harvey, a Queen’s University Belfast law professor who advocates a united Ireland. He warned the government to not unilaterally alter the constitutional fundamentals of the Good Friday agreement. Unionist opposition to Sunak’s deal has gathered steam. Two DUP MPs, Ian Paisley and Sammy Wilson, have expressed reservations. The Traditional Unionist Voice party and prominent loyalists, including Jamie Bryson, have denounced it. However, the vast majority of DUP members are yet to express an opinion. Business leaders have broadly welcomed the deal. Graham Gudgin, the chief economic adviser at the Policy Exchange thinktank and a former special adviser to David Trimble when he was first minister, urged the DUP to accept the deal and lift its boycott of power sharing, which has paralysed Stormont’s assembly and executive. “The choice is agonising and there are no good options but the best bet may be to accept what is on offer as a partial advance while continuing to campaign to remove EU law altogether,” Gudgin wrote in the Belfast News Letter. “The ongoing campaign could be pursued inside the Stormont assembly and executive. The next stage in this campaign can come in 2024 when the existing protocol provides for an assembly vote on whether the protocol should survive. The DUP will need to persuade other local parties to support major improvements to the Windsor deal.”