British Prime Minister Rishi Sunak hopes the so-called 'Stormont brake' can help win support from pro-British politicians in the region and some members of his own Conservative Party for a wider deal to overhaul post-Brexit trade rules for Northern Ireland.

Following are some details about the measure:

HOW DOES IT WORK?

In a British government document setting out details of the deal, London said the mechanism gives it an "unequivocal veto" on EU rules when 30 members of Northern Ireland's devolved government from two or more parties object.

"Once the UK notifies the EU that the brake has been triggered, the rule in question is suspended automatically from coming into effect," the document published on the British government website said.

"It can then only be subsequently applied in Northern Ireland if the UK and EU both agree to that jointly in the (UK-EU) Joint Committee," the document said, referring to the main forum for UK–EU consultation on Britain's withdrawal agreement.

"This would give the UK an unequivocal veto - enabling the rule to be permanently disapplied - within the Joint Committee."

London added that the new mechanism is not subject to European Court of Justice oversight, and that any dispute would be resolved through subsequent independent arbitration according to international and not EU law.

The Democratic Unionist Party (DUP), Northern Ireland's largest pro-British party, would have to end a year-long boycott of the regional assembly, which it started in protest at the post-Brexit rules, for the 'Stormont brake' to become a reality.

After winning 25 seats at elections last year, the DUP would need the support of five more politicians to try to trigger the brake. There are 12 other pro-British unionist politicians in the assembly now.

"MATTER OF LAST RESORT"

A separate unilateral declaration made by the UK government sets out how the mechanism would be triggered "under the most exceptional circumstances and as a matter of last resort".

Objecting local politicians would have to provide a detailed and publicly available written explanation that they have used every other available mechanism and that they had sought prior substantive discussion with the UK government and within the Northern Ireland power-sharing government.

The politicians would also have to have taken steps to consult businesses, other traders and civic society affected by the new or amended law.

If the UK accepts these conditions have been met, it would commence intensive consultations at the joint committee. In a document published on its website, the European Commission said an arbitration panel may rule on whether the conditions have been met.

It added that if the parties cannot agree either to add an amended or replacing law or to other measures, the EU can take appropriate remedial measures. This would be because of a divergence in trade rules between Northern Ireland and Ireland - and thus the broader EU, London noted.

The Commission also said the brake can only be triggered if the relevant law significantly differs in scope or content from the previous one and would have a significant impact on the everyday life of communities in Northern Ireland.

London said it cannot be "available for trivial reasons", while Irish Prime Minister Leo Varadkar told a news conference that there were "clear and strict rules" on its operation.

"The bar is set quite high," said Katy Hayward, professor of political sociology at Queen's University Belfast who has written extensively about the trade rules.

"Most of the amendments and replacements of the law are very anodyne and wouldn't really be noticed by most people. You wouldn't see it as something that would be a regular occurrence."