The decision sets Britain on a collision course with the European Union, 18 months after a trade deal that was meant to have completed Brexit.
LONDON — Britain served notice on Tuesday that it may unilaterally scrap some of the rules that govern trade with Northern Ireland, a highly political move that sets Britain on a collision course with the European Union, 18 months after a trade deal that was meant to have doused the last fires of Brexit.
Under legislation outlined by the foreign secretary, Liz Truss, the British government could discard regulations that were painstakingly negotiated with Brussels, including border checks on goods shipped from mainland Britain to Northern Ireland.
The announcement drew a quick and sharp retort from the European Union, which said that if Britain went ahead with its plans, it would respond “with all measures at its disposal,” a vague but ominous statement that could include imposing tariffs on British goods shipped across the English Channel.
Ms. Truss insisted that she wanted to change rather than completely scrap the agreement governing post-Brexit trade, known as the Northern Ireland protocol. She also said the sticking points could be resolved in negotiations with the European Union rather than with unilateral action — a gesture welcomed by Brussels.
But Ms. Truss said the protocol had disrupted trade between mainland Britain and Northern Ireland, destabilizing the delicate power-sharing agreement that has preserved peace in the North for nearly a quarter century.
The protocol is fiercely opposed by unionist parties, which favor keeping Northern Ireland as part of the United Kingdom. They complain that the rules drive a wedge between the North and mainland Britain, while the British government accuses the European Union of being overly rigid in the way it applies border checks.
“To respond to the very grave and serious situation in Northern Ireland, we are clear there is a necessity to act,” Ms. Truss said in Parliament. “We need to restore the balance in the agreement.”
“Some businesses have stopped trade altogether,” she added. “These practical problems have contributed to the sense that the east-west relationship has been undermined.”
If enacted, the law would establish a new “green channel” that would remove checks on most goods flowing west from Britain to supermarkets and shops in Northern Ireland. Those checks had been agreed upon to avoid disrupting trade between Northern Ireland, which is a member of the United Kingdom, and neighboring Ireland, which is part of the European Union, once Britain left the single market.
The Irish government also responded tartly to the announcement, saying it would undermine, rather than buttress, institutions in the North. “This unilateral action is contrary to the wishes of people and business in Northern Ireland,” said the Irish foreign minister, Simon Coveney.
Then, too, there is the risk of upsetting relations with the Biden administration and Ireland’s defenders on Capitol Hill. The White House has warned Mr. Johnson not to do anything that would jeopardize the Good Friday Agreement, the 1998 pact that ended decades of sectarian violence in the North.
Representative Richard Neal, Democrat of Massachusetts, the powerful chairman of the House Ways and Means Committee, said that any legislation that unilaterally abrogated elements of the protocol would undermine efforts to renegotiate the trade rules “at an extremely fragile time.” He suggested that Britain’s move was driven by politics.
“Northern Ireland shouldn’t be held hostage in the political process,” Mr. Neal said in a statement. “Rather all parties must stay the course and continue to work together to find durable solutions.”
Of all these tensions, a collision with Brussels would be by far the most serious, especially at a time when the British economy is suffering the soaring inflation that has afflicted other countries because of supply shocks in food and fuel. The governor of the Bank of England, Andrew Bailey, warned Monday of an “apocalyptic” rise in food prices because of shortages in wheat and other crops as a result of the war in Ukraine.
If the European Union imposed retaliatory tariffs on goods coming from Britain — a major “if” — it would attach another dragging anchor to an economy that some analysts fear is already at risk of tipping into a recession. The combination of economic stagnation and surging prices has fanned fears of a return to the dark days of the 1970s.
While the announcement seemed designed to pressure Brussels into making concessions, there were signs that it could backfire. Maros Sefcovic, vice president of the European Commission, said he was ready to continue negotiations to resolve border issues, but warned Ms. Truss that “unilateral actions contradicting an international agreement are not acceptable.”
Were she to move ahead with the bill as announced by the British government, “the E.U. will need to respond with all measures at its disposal,” he said in a statement.
“Unilateral action would only make our work on possible landing zones more difficult,” wrote David McAllister, who chairs the European Parliament’s foreign affairs committee, adding in a Twitter post that the European Union had a “unified stance.”
Others were even more scathing. Guy Verhofstadt, a European lawmaker and former Belgian prime minister, described the plan as “an attack on the international rule of law when we least need it” and a “gift” to the Russian president, Vladimir V. Putin.
Ms. Truss did not publish her proposed new legislation, the start of a long parliamentary process to make it law. British officials say that they hope talks with the European Union can continue in parallel, possibly ensuring the laws are never used.
In addition to dropping border checks, the planned law aims to give the British government unilateral powers over other sensitive areas, including tax and subsidy policy in Northern Ireland, and to curb the role of the European Court of Justice, the European Union’s top court.
Ms. Truss said she wanted a special mechanism to decide trade disputes rather than having the European Court “as the final arbiter.” While the European Union says it will try to make practical fixes over disruptive border checks, it is unlikely to agree to fundamental alterations to the protocol, such as a change to the role of the European Court.
Though the proposals were praised by hard line Brexit supporters in London, some lawmakers in Mr. Johnson’s Conservative Party expressed concern about Britain being perceived as flouting international law.
“Respect for the rule of law runs deep in our Tory veins, and I find it extraordinary that a Tory Government need to be reminded of that,” Simon Hoare, who chairs the Northern Ireland Affairs committee, said in Parliament.