LONDON — Britain and the European Union were on a collision course Thursday, after Brussels demanded the speedy withdrawal of proposed Brexit legislation that the government of Prime Minister Boris Johnson has admitted would breach international law.
The ultimatum, which Mr. Johnson’s government swiftly rejected, is the most serious crisis yet to hit negotiations on a trade agreement for after Britain leaves the European Union’s trade zone. The talks have failed to make any significant progress yet have somehow remained alive.
The dispute suggests that the moment of truth is fast approaching.
The proposed legislation would override aspects of a landmark Brexit withdrawal agreement involving the treatment of the border between Northern Ireland, which is part of the United Kingdom, and Ireland, which will remain in the European Union.
In a toughly worded statement that underscored the growing tension, the European Commission — the bloc’s executive arm — suggested it was ready to take legal action against the British government, accusing it of threatening Northern Ireland’s fragile peace process.
Mr. Johnson himself struck the withdrawal agreement with the European Union and championed it during last year’s election on his way to a landslide victory. It was crafted in part to prevent the creation of a hard border between Northern Ireland and Ireland.
But the vice president of the European Commission, Maroš Šefčovič, said Mr. Johnson’s draft bill would constitute “an extremely serious violation” of international law, and demanded its withdrawal by the end of the month. As worded, the legislation would give Britain the right to decide how to implement sensitive aspects of the treaty unilaterally rather than through negotiation with the European Union as stipulated under law.
That demand was rejected by Michael Gove, a senior British cabinet minister, who said the British government had made it “perfectly clear” it would not withdraw the bill even though it has admitted that it breaks international law in a limited way.
In issuing its ultimatum, the European Union stopped short of taking an irreversible step or walking away from the trade talks. And the two sides have agreed to keep on talking next week after what David Frost, Britain’s main negotiator, described as “useful exchanges” that took place separately on Thursday.
He and his counterpart, Michel Barnier, need to reach a deal on trade next month if it is to be ratified by the end of the year when Britain stops trading under European Union’s rules.
“With the E.U. saying ‘you have until the end of the month,’ one side has to back down,” said David Henig, director of the UK Trade Policy Project at the European Center for International Political Economy, a research institute, “and that is a difficult situation if we are to get a deal by mid-October. The E.U. is not going to back down.”
There is no sign Mr. Johnson plans to back down to either, claiming earlier this week that failing to reach a trade deal with the European Union would still be a “good outcome.” Though he has shown flexibility in the past, and his government has made a series of policy reversals, the government has taken an uncompromising line on Brexit, giving few hints that it intends to compromise, let alone give way to Brussels.
While another crisis in Brexit negotiation seemed almost inevitable, few expected the confrontation to be about putting in place an agreement that had already been signed and sealed. Britain argues that its new draft legislation is to provide a fallback option in case it cannot strike a trade deal with Brussels, but it not even clear that it will pass through Parliament unscathed.
Critics said the risk of miscalculation for Mr. Johnson was high. Far from being a peripheral issue, the provisions on the Irish border are the centerpiece of the withdrawal agreement. They united the bloc’s 27 members during protracted, divisive negotiations with Britain over Brexit, because they are seen as so closely intertwined with maintaining Irish peace.
“The E.U. is not going to allow peace in Ireland to be leveraged in a negotiation,” said Mujtaba Rahman, an expert on Brexit at the political risk consultancy, Eurasia Group. “They see very clearly what the government is doing, and they’re not going to be bullied. Britain is playing a very dangerous game of chicken with Europe.”
There are other international consequences to the gamesmanship.
Democratic leaders in Washington warned Mr. Johnson that he was putting his hopes for a trans-Atlantic trade deal at risk. Undermining the Northern Ireland provisions in the withdrawal agreement, they said, would jeopardize the Good Friday Accord, a peace deal brokered under President Bill Clinton that ended decades of sectarian violence in Ireland.
“If the U.K. violates that international treaty and Brexit undermines the Good Friday Accord, there will be absolutely no chance of a U.S.-U.K. trade agreement passing the Congress,” Speaker Nancy Pelosi of California said in a statement on Wednesday.
Representative Richard E. Neal, a Massachusetts Democrat who is chairman of the powerful House Ways and Means Committee, urged Britain to uphold the withdrawal agreement.
“I sincerely hope the British government upholds the rule of law and delivers on the commitments it made during Brexit negotiations, particularly in regard to the Irish border,” Mr. Neal said in a statement.
“Every political party on the island opposes a return of a hard border,” he said.
The Democratic nominee, former Vice President Joseph R. Biden Jr., is a staunch defender of Ireland who said he would have voted against Brexit if he were a British citizen. One of his top foreign policy advisers, Antony Blinken, suggested in a tweet this week that Mr. Biden was watching the situation carefully.
Mr. Biden, he wrote, “is committed to preserving the hard-earned peace & stability in Northern Ireland.” He added, “As the UK and EU work out their relationship, any arrangements must protect the Good Friday Agreement and prevent the return of a hard border.”