LONDON >> Senior U.K. and European Union officials met Thursday as part of what Britain called “intensive negotiations” to resolve a thorny post-Brexit trade dispute that has spawned a political crisis in Northern Ireland.

Northern Ireland Secretary Chris Heaton-Harris traveled to Brussels to meet Maros Sefcovic, the EU’s chief Brexit negotiator.

Hopes of a breakthrough were raised last week when Prime Minister Rishi Sunak flew to Belfast to meet leaders of Northern Ireland’s political leaders and discuss progress. But hints of compromise by the U.K. government sparked opposition from hard-line euroskeptics in Sunak’s governing Conservative Party, costing the prime minister vital political momentum to secure a deal.

The Democratic Unionist Party, Northern Ireland’s main British unionist force, also has warned it will oppose any deal that does not meet its demand for “significant, substantive change” to the Brexit treaty between the U.K. and the EU.

“We are not just talking about tinkering around the edges,” DUP leader Jeffrey Donaldson said Wednesday.

Northern Ireland is the only part of the U.K. that shares a border with an EU member nation, the Republic of Ireland. When the U.K. left the bloc, the two sides agreed to keep the Irish border free of customs posts and other checks because an open border is a key pillar of Northern Ireland’s peace process.

The fragility of that peace was underscored by the shooting of an off-duty senior Northern Ireland police officer on Wednesday as he coached children at soccer. Police said Irish Republican Army dissidents were suspected in the attack, which left the officer in critical condition.

Under the agreement, there are checks on some goods entering Northern Ireland from the rest of the U.K. British unionist politicians argue that the new trade border undermines Northern Ireland’s place in the United Kingdom. The DUP walked out of Northern Ireland’s power-sharing government a year ago in protest and is refusing to return until the rules, known as the Northern Ireland Protocol, are redrawn.

The protocol has become a huge political headache for the U.K. government and a thorn in the country’s relations with the EU. The two sides have bickered since the divorce became final in 2020, with Britain introducing a bill that would let it unilaterally rip up parts of the Brexit agreement, and the EU accusing the U.K. of failing to honor the legally binding treaty it had signed.

The mood has improved since Sunak took office in October, replacing the more belligerent Liz Truss and, before her, Boris Johnson.

But trying to fix the Brexit deal is a big gamble for Sunak. Pro-Brexit Conservatives brought down Prime Minister Theresa May in 2019 after repeatedly rejecting her attempts to forge a closer relationship with the EU, and they could hand Sunak the same fate if he pushes through an agreement without the Brexiteers’ support.

The thorniest issue is the role of the European Court of Justice in resolving any disputes that arise over the rules. That role was agreed by Britain and the EU in the Brexit divorce deal, but the DUP and Conservative Brexiteers insist the European court must have no jurisdiction in U.K. matters.

The U.K. hopes to resolve the dispute and restore Northern Ireland’s government before the 25th anniversary in April of Northern Ireland’s 1998 Good Friday peace accord. The British government hopes U.S. President Joe Biden will visit to mark the peace milestone.

Sunak told lawmakers on Wednesday that “I have a good understanding of what is required and I will keep fighting until we get it.”

But if he fails he could become another Conservative prime minister brought down by Brexit.