Theresa May has launched a blistering attack on the government’s plan to give itself powers to renege on the special arrangements for Northern Ireland in the Brexit deal. She described the plans as “reckless” and “irresponsible” and said they “risked the integrity of the United Kingdom”, as they would not only tarnish Britain’s reputation globally as an upholder of the law but could contribute to a reunited Ireland. In a strongly worded speech in the House of Commons, she said: “I cannot emphasise enough how concerned I am [that] the Conservative government is willing to go back on its word to break an international agreement signed in good faith, and to break international law.” Sitting beside two former Conservative party Northern Ireland secretaries, Karen Bradley and Theresa Villiers, and Sir Bob Neill, who threatened a backbench rebellion against the government, she said there could never be a time a minister could walk through the voting lobbies and say yes to breaking the law. May said the bill would also mean trust would be undermined in future negotiations with other countries. “So much for global Britain,” she quipped. The Belfast South MP, the SDLP’s Claire Hanna, said Boris Johnson’s government had to own the consequences of the type of Brexit it agreed and should not “feign shock” when nine months later it emerged there would be trade barriers between east and west. “The government is acting recklessly and irresponsibly with no thought to the long-term impact on the standing of the United Kingdom in the world. This will lead to untold damage to the United Kingdom’s reputation. It puts the future of the United Kingdom at risk. And, as a result, with regret, I have to tell the minister I cannot support this bill,” she said. The bill, which is designed to regulate trade within the UK after Brexit, was unveiled by Downing Street two weeks ago, catching Conservative MPs, opposition parties and the EU off guard. A backbench rebellion, now thought to have been brought under control by Downing Street, emerged when it became clear the bill included three clauses that would allow the government to disapply parts of the Northern Ireland protocol without regard to the co-signatories of an international treaty. The Northern Ireland Office minister Robin Walker defended the clauses, saying they were needed to ensure nothing in the protocol of the withdrawal agreement “prevents unfettered access from Northern Ireland to the rest of the UK”. But May hit back and said there were three reasons why clauses 41 to 45 had “no place” in the bill and could weaken support for Westminster in Northern Ireland. “That, in turn will lead to some communities having less willingness to trust the United Kingdom government and that would have a consequence on the willingness of people in Northern Ireland to remain part of the United Kingdom,” she said. “So, far from acting to reinforce the integrity of the United Kingdom, in pursuit of trying to appear to be tough to the European Union, I think the government is putting the integrity of the United Kingdom at risk,” the former prime minister added. The former Tory party leader Iain Duncan Smith said he did “not believe a word” of claims that breaking international law would undermine trust in the UK because, he said, the EU itself broke international law.