Senior Conservative MPs and a raft of legal experts have urged the government not to go ahead with plans to drive through a change to the Brexit agreement on Northern Ireland after a minister conceded it would break international law. The remarkable admission by Brandon Lewis, secretary of state for Northern Ireland, followed the resignation of the most senior legal civil servant and has raised questions over the future of justice secretary, Robert Buckland, and attorney general, Suella Braverman, both of whom have taken oaths to uphold the rule of law. Their positions were put in the spotlight after the departure of Sir Jonathan Jones, who is understood to have become exasperated that ministers intended to ignore his advice that any changes to the new internal market bill would likely be illegal. The Guardian understands the government sought independent legal advice from a leading barrister, whose advice chimed with that of Jones, but their opinions were overridden by ministers. One EU diplomat suggested the minister’s admission the UK was prepared to break international law could have an immediate impact on the current talks in London between UK chief negotiator David Frost and the EU’s Michel Barnier. “If true, it would be a massive blow to the UK’s international reputation and have huge negative consequences on the current talks with the EU”, the source said. “It would be in Britain’s best interests to clarify its plans now, urgently, and assure the EU that it will continue to honour its commitments under the withdrawal agreement under all circumstances. Who would want to agree trade deals with a country that doesn’t implement international treaties? It would ultimately be a self-defeating strategy.” The Guardian has been told that Jones, the head of the UK government’s legal department, disagreed with Braverman’s initial interpretation of the legal implications of a no-deal Brexit. This raised questions about whether government plans to override the Brexit withdrawal agreement were in breach of the ministerial code, which obliged ministers to follow the law, including international law. A Whitehall source said Jones was also aware that stepping outside the law would be a breach of the civil service code, by which he is expected to abide. Jones’s departure came just hours before Lewis astonished the House of Commons. He told MPs: “Yes, this does break international law in a very specific and limited way. We’re taking the powers of this to apply the EU law concept of direct effect … in a certain very tightly defined circumstance.” The former prime minister Theresa May questioned whether Johnson was risking the UK’s international reputation. “How can the government reassure future international partners that the UK can be trusted to abide by the legal obligations of the agreement itself,” she said in an exchange with Lewis. Furious Tory MPs have urged the government to reconsider pursuing a breach of international law, including the Conservative chairs of three select committees. Bob Neill, the chair of the justice select committee, said: “Any breach, or potential breach, of the international legal obligations we have entered into is unacceptable, regardless of whether it’s in a ‘specific’ or ‘limited way’. Adherence to the rule of law is not negotiable.” Concerns were also raised by Tom Tugendhat, foreign affairs chair, and by Tobias Ellwood, who chairs the defence committee. Tugendhat said: “Our entire economy is based on the perception that people have of the UK’s adherence to the rule of law. I hope it’s clear where I stand on that.” The Welsh government’s minister for European transition, Jeremy Miles, described the bill as an “attack on democracy” and a plan to “sacrifice the future of the union by stealing powers from devolved administrations”. Sir Bernard Jenkin, the senior Brexiter and chair of the House of Commons’ liaison committee, said he was shocked by Lewis’ statement to MPs and suggested he may have made an error. “I was astonished that a minister of the Crown should say to the House of Commons that we will attempt to breach international law,” he told the Guardian. “I don’t think he has got that quite right. This is about insisting on a shared interpretation of an agreement that we have made with the EU. The EU should no more insist on their interpretation than we should insist on ours.” He said that the government can still challenge the EU over its interpretation of the withdrawal agreement without telling Parliament that the UK will ignore international legislation. “I think Brandon Lewis misspoke and got the emphasis wrong. The EU does not have a monopoly of wisdom over the meaning of the agreement,” he said. Other public critics included the veteran Tory Roger Gale who said attempts to re-write the protocol “will be regarded worldwide as an act of bad faith”. The former minister George Freeman said the supreme court would be “preparing to remind ministers that intentionally breaking the law – even in a very specific and limited way – is, well, unlawful.” Lord Falconer, the shadow attorney general, said the positions of Braverman and Buckland, as members of a government that has admitted it intended to break the law but who had taken an oath to uphold the rule of law, should now be put into question. Dinah Rose, the barrister and president of Magdalen College, Oxford, said: “If a barrister advises a client that a particular act would be unlawful, but the client insists on doing it anyway, the barrister may not continue to represent that client. The barrister is professionally embarrassed and should resign. I’m sure Robert Buckland knows this.” Government sources claimed the move would not breach the ministerial code because the obligation on ministers to comply with international law was removed from the guidance in 2015 by David Cameron, although they conceded there was some disagreement over the interpretation. However legal experts including Lord Anderson, former terror watchdog and member of the House of Lords justice committee, said the code still mandated ministers to uphold international law following a court of appeal ruling in 2018, which concluded there was an “overarching” duty of ministers to comply with international law. Trade expert David Henig, director of the European Centre for International Political Economy thinktank, said it looks like “there are issues” over the Northern Ireland protocol, while law experts suggested it was an attempt to re-write the rules on state aid in Northern Ireland. In a statement from the Northern Ireland Office, the government is promising to deliver “true unfettered access … without paperwork” for trade between Northern Ireland and Great Britain. It also said that “while Northern Ireland will remain subject to the EU’s state aid regime for the duration of the protocol, GB will not be subject to EU rules in this area.” It is understood Lewis did not misspeak in the Commons and senior government figures had decided it was better to be honest that the plans would breach international law – rather than stonewall. Jones becomes the sixth senior Whitehall figure to resign in recent months, including the cabinet secretary, Mark Sedwill, the Foreign Office permanent secretary, Simon McDonald, and Jonathan Slater from the Department of Education. Sir Philip Rutnam, former permanent secretary at the Home Office, has begun legal proceedings against the department for constructive dismissal. Pro-Brexit Conservative MPs stepped up their own lobbying on Tuesday, urging Johnson to go further and to ditch the EU withdrawal agreement entirely should no trade deal be reached, claiming they were promised he would do so before they passed the bill in January. Sir Bernard Jenkin, who chairs the European Research Group of hard-Brexit Tory MPs, said the group of MPs who destroyed May’s deal had only voted in favour of Johnson’s because they had been assured the government would ditch the agreement if no trade deal was reached with the European Union. “We made clear, however, that this agreement was barely ‘tolerable’ and only voted for it against assurances given by government: that it was just a starting point for negotiations; that it would be superseded by a full FTA [free trade agreement]; and, if needs be, could be repudiated,” he wrote in the Diplomat magazine. A Downing Street spokesman said they “remain committed to the implementation of the withdrawal agreement and the Northern Ireland protocol” regardless of whether a deal was reached.