LONDON (AP) — The U.K. and the European Union ended years of wrangling and acrimony on Monday, sealing a deal to resolve their thorny post-Brexit trade dispute over Northern Ireland.

Prime Minister Rishi Sunak and European Commission President Ursula von der Leyen signed off on the deal at a meeting in Windsor, England. Afterwards Sunak said the pair would hold a news conference on Monday. The government had previously said that would only happen if a deal was struck.

The agreement is be a big victory for Prime Minister Rishi Sunak — but not the end of his troubles. Selling the deal to his own Conservative Party and its Northern Ireland allies may be a tougher struggle. Now Sunak awaits the judgment of Northern Ireland's Democratic Unionist Party, which is boycotting the region's power-sharing government until the trade arrangements are substantially changed.

Sign of a potential U.K.-EU breakthrough came Sunday, when the two sides announced that von der Leyen would meet Sunak in Windsor, about 20 miles (32 kilometers) west of London. Sunak is due to make a statement by Sunak to the House of Commons later.

A meeting between von der Leyen and King Charles III at Windsor Castle is also planned. The meeting is likely to touch on a variety of issues including climate change and the war in Ukraine.

Buckingham Palace said the meeting was taking place on the government's advice, leading critics to accuse Sunak of dragging the monarch, who is supposed to remain neutral, into a political row.

"I cannot quite believe that No. 10 would ask HM the King to become involved in the finalising of a deal as controversial as this one. It's crass and will go down very badly in NI," former Northern Ireland First Minister Arlene Foster said on Twitter.

Sunak's spokesman, Max Blain, said the government "would never" embroil the king in politics.

"His Majesty has met with a number of foreign leaders recently," he said, including Polish President Andrzej Duda and Ukraine's Volodymyr Zelenskyy. "This is no different."

If all goes to plan, the deal could end a dispute that has soured U.K.-EU relations, sparked the collapse of the Belfast-based regional government and shaken Northern Ireland's decades-old peace process.

Northern Ireland is the only part of the U.K. that shares a border with an EU member, the Republic of Ireland. When the U.K. left the bloc in 2020, 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 undermines Northern Ireland's place in the United Kingdom.

The Democratic Unionist Party collapsed Northern Ireland's Protestant-Catholic power-sharing government a year ago in protest and has refused to return until the rules are scrapped or substantially rewritten.

The DUP has stayed largely silent in recent days, saying it needs to see the details of a deal before deciding whether it meets the party's self-imposed tests.

Hints of compromise towards the EU also have sparked opposition from hard-line euroskeptics who form a powerful bloc in Sunak's governing Conservative Party. Critics include former Prime Minister Boris Johnson, who as leader at the time of Brexit signed off on the trade rules that he now derides. Johnson was ousted by the Conservatives last year over ethics scandals, but is widely believed to hope for a comeback.

Jacob Rees-Mogg, a prominent pro-Brexit Tory lawmaker, said acceptance of any deal "will all depend" on the DUP. "If the DUP are against it, I think there will be quite a significant number of Conservatives who are unhappy," Rees-Moog said.

In a boost for Sunak's chances of winning Conservative support, lawmaker Steve Baker — a self-styled "Brexit hardman" who helped topple Prime Minister Theresa May by opposing her Brexit deal in 2019 — said Sunak was "on the cusp of securing a really fantastic result."

Sunak has said Parliament will get to debate any deal he strikes, but he hasn't promised lawmakers a binding vote on it, and no vote in Parliament is expected this week.

Relations between the U.K. and the EU, severely tested during the long Brexit divorce, chilled still further amid disputes over the Northern Ireland Protocol. The U.K. government introduced a bill that would let it unilaterally rip up parts of the Brexit agreement, a move the EU called illegal. The bloc accused the U.K. of failing to honor the legally binding treaty it had signed.

The mood between London and Brussels improved after Sunak, a pragmatic Brexit supporter, took office in October, replacing more belligerent predecessors — Johnson and Liz Truss.

A deal is likely to remove customs checks on the vast majority of goods moving between the U.K. and Northern Ireland and to give Northern Ireland lawmakers some say over EU rules that apply there as part of the Protocol.

The thorniest issue is the role of the European Court of Justice in resolving any disputes that arise over the rules.

The U.K. and the EU agreed in their Brexit divorce deal to give the European court that authority. But the DUP and Conservative Brexiteers insist the court must have no jurisdiction in U.K. matters.

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