LONDON — Prime Minister Boris Johnson’s Brexit negotiations fell into disarray on Tuesday, as the government’s top lawyer resigned over Mr. Johnson’s plan to override a landmark agreement with the European Union, and one of his own ministers admitted that the changes would break international law.
The head of the government’s legal department, Jonathan Jones, resigned abruptly, the day before the government planned to introduce legislation that would rewrite provisions on the treatment of Northern Ireland, should Britain fail to strike a permanent trade agreement with Brussels by the end of this year.
Mr. Johnson’s aggressive move to pull back from the agreement about Northern Ireland underscored his determination for Britain to control its own economic destiny — even at the cost of triggering another confrontation with the European Union, shredding his own diplomacy and raising questions about his government’s commitment to the rule of law.
It carried echoes of the moment he suspended Parliament last fall, effectively squelching a debate over his drive to complete Brexit, a maneuver that Britain’s Supreme Court later declared unlawful.
Mr. Jones did not detail his reasons for resigning, but The Financial Times, which first reported the news, said he clashed with officials in Downing Street about plans to rewrite the so-called Northern Ireland protocol, a central plank of the Withdrawal Agreement, under which Britain left the European Union last January.
The prospect of Britain reneging on a treaty that Mr. Johnson himself signed ignited a firestorm in Parliament, with even the prime minister’s Conservative predecessor, Theresa May, sharply criticizing the government.
“How can the government reassure future international partners that the U.K. can be trusted to abide by the legal obligations of the agreements it signs?” said Ms. May during a tense debate in the House of Commons. She negotiated the bulk of the Withdrawal Agreement with Brussels.
Mr. Johnson’s Northern Ireland secretary, Brandon Lewis, insisted that the proposed changes were not intended to rip up the agreement but merely to create a “safety net” for Northern Ireland businesses in the event that London could not work out long-term trading arrangements with Brussels.
But he acknowledged that the changes would “break international law in a very specific and limited way.” Mr. Lewis argued there was a precedent for Britain to reconsider its obligations when circumstances change, drawing catcalls from opposition lawmakers and incredulous looks, even from some Conservatives.
“One of the things the U.K. has always prided itself on is upholding the rule of the law,” said Professor Catherine Barnard, an expert on European Union law at Cambridge University. “To have a minister of state stand up and say, ‘We’re breaking international law, but only a little’ is extraordinary.”
Professor Barnard said there were legal contradictions in the agreement, having to do with when customs duties must be imposed on goods shipped from Britain to Northern Ireland. But she said the agreement provided ways to resolve these issues and Britain had no right to change it unilaterally.
The trade negotiations, which turn on broader issues like Britain’s ability to direct state subsidies to its companies, resumed on Tuesday in London against an already acrimonious backdrop. Mr. Johnson declared that a no-deal Brexit would be a “good outcome” for Britain and that the Withdrawal Agreement, which he once hailed as a triumph, was a flawed deal that needed to be fixed.
Mr. Johnson’s reversal seemed driven in part by a desire to mollify the hard-line Brexiteers in his party. They have long agitated for Britain to negate the deal on Northern Ireland, complaining that it impinges on British sovereignty.
Under the agreement, Northern Ireland would remain part of Britain’s customs territory but would abide by European Union rules on issues ranging from safety standards to state subsidies to industry.
European Union leaders expressed alarm that Britain would go back on an international treaty, ratified by all the members of the bloc. Ursula von der Leyen, the president of the European Commission, warned Britain that a future trade agreement depended on it carrying out this one.
The resignation of Mr. Jones, the latest in a parade of civil servants who have left the Johnson government, left little doubt that the government’s actions raised thorny legal problems. A 58-year-old barrister who served in the Home Office and the attorney general’s office, Mr. Jones rose to become the government’s senior lawyer, rendering judgment on the legality of new legislation.
He is the sixth senior civil servant to quit since Mr. Johnson won a landslide election victory last December and his chief political adviser, Dominic Cummings, embarked on a campaign to shake up Britain’s bureaucracy.
“When your top lawyer resigns, it sends an extraordinary signal,” said Bobby McDonagh, who served as the Irish ambassador to Britain.
Britain’s move to rewrite the agreement, he said, suggested the Johnson government did not fully understand the implications of the language its own diplomats agreed to. “They’re really amateurs when it comes to trade negotiations,” Mr. McDonagh said.
The damage to Britain’s international reputation could be substantial, at a time when it is trying to negotiate trade agreements, not just with the European Union but also with the United States.
In Washington, Congressional Democrats, as well as the Democratic presidential nominee, former Vice President Joseph R. Biden Jr., are likely to view any changes in Britain’s treatment of Northern Ireland as jeopardizing the Good Friday Agreement, which ended decades of sectarian violence there.
Some analysts said they viewed Mr. Johnson’s moves as a way to increase his leverage with the European Union as the trade talks enter a make-or-break period. But others said the legislation underscored that members of his government were determined to diverge as sharply as possible from the bloc.
The biggest bone of contention is the European Union’s demand that Britain abide by its rules on state subsidies to businesses. Britain has yet to offer a counterproposal, and Mr. Johnson’s aides are determined to keep control over industrial policy so they can pour money into high-tech sectors like biotechnology and artificial intelligence.