A majority of Northern Ireland assembly members have sent a damning joint letter to Boris Johnson condemning his plan to override the Northern Ireland protocol, saying it was “a fabrication” for the prime minister to claim he was trying to protect the region. The letter, signed by 52 of the 90 members of the devolved assembly elected in May, said the government’s bill to unilaterally change the protocol “flies in the face of the expressed wishes of not just most businesses but most people in Northern Ireland”. Downing Street has insisted the government received advice on whether a one-sided attempt to change the post-Brexit trade protocol risked breaching international law, although it plans to release only a summary of this when the bill is published on Monday. Johnson characterised the bill on Monday as a “bureaucratic change” designed to unify Northern Irish communities and protect the 1998 Good Friday peace agreement. But the letter, signed by assembly members from the Social Democratic and Labour party (SDLP), Sinn Féin and the Alliance party said that while the protocol had flaws, it “currently represents the only available protections for Northern Ireland” from the impacts of Brexit. “The protocol also offers clear economic advantages to our region, and the opportunity for unique access to two major markets,” it went on. “The fact that you have removed this advantage from businesses in Great Britain, at a clear economic cost, does not justify doing the same to businesses in Northern Ireland.” It said the way to improve the arrangements was through engagement with the EU based on trust, “rather than law-breaking and unilateral abrogation of treaty obligations”. The letter continued: “It is also deeply frustrating that you and your ministers continue to misrepresent our desire to see smooth implementation as an endorsement of your government’s reckless actions on the protocol – it is categorically not. “To complain the protocol lacks cross-community consent, while ignoring the fact that Brexit itself – let alone hard Brexit – lacks even basic majority consent here, is a grotesque act of political distortion.” Johnson’s argument that he was acting to protect Northern Irish institutions was “as much a fabrication as the Brexit campaign claims you made in 2016”, the letter said, promising to oppose the bill. Johnson rejected the idea that the bill would break international law, telling LBC Radio: “I disagree with that. Why? Because I think our higher and prior legal commitment as a country is to the Belfast Good Friday agreement, and to the balance of stability of that agreement.” He said the bill, reportedly toughened up in scope as a result of pressure from strongly pro-Brexit ministers and MPs, was “the right way forward”. “One community feel very, very estranged from the way things are operating, very alienated,” he said. “We’ve just got to fix that. It is relatively simple to do it. It’s a bureaucratic change that needs to be made. Frankly, it’s a relatively trivial set of adjustments in the grand scheme of things.” The bill is expected to unilaterally change elements of the protocol, which set out post-Brexit trade rules connected to Ireland, Northern Ireland and the rest of the UK. It is expected to set out a dual regulatory regime, allowing Northern Ireland businesses to keep to either UK or EU standards, and scrap checks on goods arriving into Northern Ireland from Great Britain if they are not then moving across the Irish border. A readout from Ireland’s foreign ministry of a call between the country’s foreign minister, Simon Coveney, and his UK counterpart, Liz Truss, took a notably different tone, calling the bill “deeply damaging to relationships on these islands and between the UK and EU”. It added: “Minister Coveney said it marks a particular low point in the UK’s approach to Brexit, especially as Secretary Truss has not engaged with negotiations with the EU in any meaningful way since February. Minister Coveney repeated that the protocol is the negotiated solution, ratified by Westminster, to the hard Brexit pursued by the UK government.” The readout said the call lasted just 12 minutes.