LONDON (AP) — U.K. Prime Minister Rishi Sunak traveled to Belfast on Tuesday to sell his landmark agreement with the European Union to its toughest audience: Unionist politicians who fear post-Brexit trade rules are weakening Northern Ireland's place in the United Kingdom.

The U.K. and the 27-nation EU announced Monday that they had struck a deal to resolve a dispute over Northern Ireland trade that has vexed relations since the U.K. left the bloc in 2020. The agreement will ease customs checks and other hurdles for goods moving to Northern Ireland from the rest of the U.K. that were imposed after Brexit to maintain an open border between the north and its EU neighbor, the Republic of Ireland.

The deal, dubbed the "Windsor Framework," was hailed by London and Brussels as a breakthrough. But Northern Ireland's British unionist politicians have yet to give it their blessing. Their support is key to restoring Northern Ireland's semi-autonomous government, which has been toppled by the trade feud, leaving 1.9 million people without a functioning administration.

Sunak told the BBC the deal was "a huge step forward for the people of Northern Ireland" and he was confident politicians there would support it.

The Democratic Unionist Party, which had governed alongside Irish nationalists Sinn Fein in the power-sharing administration, walked out of the government a year ago to protest the trade rules and has refused to return until they are scrapped or substantially rewritten. Under Northern Ireland's political system, power is shared between Irish nationalists and British unionists, and neither side can govern without the other.

DUP leader Jeffrey Donaldson said Tuesday that the party would "take our time" to pore over the details of the deal before delivering its verdict.

"We're reasonable people, but we want to ensure that what the prime minister has said is matched by what is actually in the agreement itself," he said.

Northern Ireland is the only part of the U.K. that shares a border with an EU member. 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.

Instead, there are checks on some goods entering Northern Ireland from the rest of the U.K. That angered British unionist politicians in Belfast, who say the new trade border in the Irish Sea undermined Northern Ireland's place in the United Kingdom.

Sunak said the new Windsor Framework "removes any sense of an Irish Sea border," lifting checks on the vast majority of goods. It also gives Northern Ireland politicians a mechanism to challenge new EU trade rules that could apply in the region — a key unionist demand.

The deal marks a turn for the better in relations between Britain and the EU, which have always been scratchy and grew acrimonious after Brexit. The mood between London and Brussels improved after the pragmatic Sunak took office in October, replacing more belligerent predecessors — Boris Johnson and Liz Truss.

Sunak brought a change in tone, and injected new momentum into attempts to ease trade friction. U.K. and EU negotiators worked for weeks, sometimes late into the night in windowless rooms in Brussels, to move beyond entrenched positions that had ground to stalemate.

The U.K. says the EU's acceptance of the "Stormont Brake" emergency mechanism - which gives Northern Ireland politicians a way to challenge future EU goods rules - was a major breakthrough.

Business groups largely welcomed the deal, saying it would ease the burdens faced by companies and give Northern Ireland customers access to goods such as English sausages that were blocked under the original post-Brexit rules.

Some hard-line pro-Brexit politicians in Sunak's governing Conservatives also were surprisingly positive about the deal. One key figure has yet to weigh in — former Prime Minister Boris Johnson, who negotiated the original Brexit divorce deal that Sunak has now rewritten.

The agreement was also welcomed by the White House, which said it was "grateful" the two sides could resolve the dispute.

"We believe that this will help the prosperity of both the EU and the U.K. and will open up all kinds of … avenues for trade that were somewhat at risk," John Kirby, a spokesman for the National Security Council, said Monday.