(AFP) - Britain and the European Union on Monday agreed a crucial overhaul of trade rules in Northern Ireland, a breakthrough aimed at resetting seriously strained relations since Brexit.

Prime Minister Rishi Sunak and European Commission president Ursula von der Leyen adopted the deal at talks in Windsor, west of London, both sides said.

Their meeting followed more than a year of tense negotiations over the "Northern Ireland Protocol", which has unsettled the province 25 years on from a historic peace deal that ended three decades of armed conflict.

Agreed in 2020 as part of Britain's EU divorce, the pact kept the province in the European single market for physical goods and subject to different customs rules than the rest of the UK, angering pro-UK unionists there and eurosceptics in London.

The UK government had threatened a unilateral overhaul of the protocol unless the EU agreed to wholesale changes, souring diplomatic ties and risking a wider trade war, but that prospect now appears to be receding.

"I'm looking forward to turning a page and opening a new chapter with our partner and friend," von der Leyen said as she left Brussels ahead of the talks.

The EU chief was also set to meet King Charles III while in Windsor, stoking accusations in the UK that Sunak was trying to project royal endorsement of the expected deal.

It is likely to face opposition from Brexiteers, including Sunak's potentially rebellious predecessor Boris Johnson, and from lawmakers representing the pro-British unionist community in Northern Ireland.

Sunak's spokesman insisted the monarch's meeting with von der Leyen was decided by Buckingham Palace.

- Power-sharing -

Sunak and von der Leyen, who met at the Fairmont Hotel in Windsor, are due to hold a short mid-afternoon press conference. The UK leader -- who only took power in October -- will then make a statement to parliament scheduled for 6:30 pm (1830 GMT).

The agreement ends a long chapter of talks between London and Brussels, under the direction of three different British prime ministers and the cloud of the war in Ukraine.

It is seen as long overdue to help stabilise both Northern Ireland and the wider UK's post-Brexit relationship with its European partners.

The protocol has faced staunch opposition from the Democratic Unionist Party (DUP), the largest pro-UK party in Northern Ireland, which argues that it threatens the province's place within the UK.

London has been pressing Brussels to agree a "green", check-free lane for goods coming from the rest of the UK that are intended to stay in Northern Ireland, without heading into Ireland and the EU's single market.

The deal would also reportedly limit, but not scrap, oversight of the protocol by the EU's European Court of Justice.

The DUP is particularly angered by the prospect of EU law retaining a role in Northern Ireland, and its response in turn could determine how Conservative eurosceptics in London react.

DUP leader Jeffrey Donaldson, who has been refusing to re-enter a power-sharing government in Belfast set to be led by pro-Irish nationalists, tweeted that the party would "take our time to consider the detail".

Former cabinet minister and Johnson loyalist Jacob Rees-Mogg told ITV: "I'm not sure he (Sunak) has achieved the objective of getting the DUP back into power-sharing, which is the fundamental point of it."

- 'About time' -

The UK, which is grappling with low economic growth and its worst cost-of-living crisis in a generation, is seen as eager to reset relations to boost trade.

The government in London is also under pressure to restore power-sharing in Belfast, with the 25th anniversary of the 1998 Good Friday Agreement looming large.

Northern Ireland has been without a devolved government since February last year due to the DUP's boycott.

In the Northern Irish border city of Newry, some residents were eager for a breakthrough and the restoration of power-sharing.

"We need things to get going again, we need to get this sorted out," Vincent Ward, 53, told AFP.

Joe O'Hanlon, 63, added it was "about time" that elected leaders "got their act together".

© Agence France-Presse