Britain says its move to change the legally binding treaty — an apparent breach of international law — is an insurance policy in case it can’t reach an agreement with the bloc to end a long-running dispute over post-Brexit trade rules.

“Our preference is to reach a negotiated outcome with the EU,” said Foreign Secretary Liz Truss.

The announcement drew a sharp response from the EU, which has long accused Prime Minister Boris Johnson of trying to wriggle out of a deal that his government negotiated and signed as part of the UK’s exit from the bloc in 2020.

The spat raises the chances of a trade war between Britain and the 27-nation bloc that is — even after Brexit — its major economic partner.

Britain's Conservative government says post-Brexit trade rules — which London signed up to — are hurting the economy and undermining peace in Northern Ireland, the only part of the UK that shares a border with an EU member state.

When Britain left the bloc and its borderless free-trade zone, a deal was agreed to keep the Irish land border free of customs posts and other checks, because an open border is a key pillar of the peace process that ended decades of violence in Northern Ireland.

Instead, to protect the EU’s single market, there are checks on some goods, such as meat and eggs, entering Northern Ireland from the rest of the UK.

The arrangement is opposed by British unionists in Northern Ireland, who say the new checks have put a burden on businesses and frayed the bonds between Northern Ireland and the rest of the UK.

The Democratic Unionist Party, Northern Ireland’s biggest unionist party, is blocking the formation of a new power-sharing regional government in Belfast until the checks are scrapped.

The British government agrees that the trade regulations, known as the Northern Ireland Protocol, are destabilizing a peace agreement that relies on support from both Protestant unionist and Catholic nationalist communities.

While the DUP wants the Protocol scrapped, most other parties in Northern Ireland want to keep it, with tweaks to ease the burden on businesses.

Johnson says his government wants to fix, rather than scrap, the arrangements, using technology and trusted-trader programs to create a check-free “green lane” for goods destined for Northern Ireland that are at little risk of entering the EU.

The British government hopes its threat of legislation — which would take months to pass through Parliament — will increase pressure on the EU to compromise.

The bloc insists the treaty can’t be renegotiated, though it is willing to be flexible about how the rules are implemented. The EU could hit back with legal action, and potentially trade sanctions, if Britain does not back down.

Critics of Johnson's government say a UK-EU feud is the last thing Europe needs as it seeks unity in response to Russia’s invasion of Ukraine, and risks damaging the British economy amid a worsening cost-of-living crisis.

“This is not a time for posturing or high-stakes brinksmanship,” said Labour Party lawmaker Stephen Doughty.

“The prime minister negotiated this deal, signed it, ran an election campaign on it. He must take responsibility for it and make it work.”

Johnson denied the move breached international law, saying Britain’s “higher duty” was to uphold the Good Friday Agreement, Northern Ireland’s 1998 peace accord.

But some Conservatives expressed misgivings.

“Respect for the rule of law ... runs deep in our Tory veins,” said Conservative legislator Simon Hoare. “I find it extraordinary that a Tory government needs to be reminded of that.”