A day before the UK is set to unveil new legislation on Northern Ireland, the head of Irish Sinn Fein party, Mary Lou McDonald said the changes would "undoubtedly" breach international law.

The UK government has remained tight-lipped on the details of its plan. But the UK's Northern Ireland Secretary Brandon Lewis told British broadcaster Sky News that the changes would be "lawful and correct."

Northern Ireland Protocol — a source of discord

The legislation, due to come into force on Monday, will affect the existing Northern Ireland protocol negotiated by the EU and the government led by current Prime Minister Boris Johnson after Britain officially left the bloc in 2020. The accord allows for the Republic of Ireland to maintain a de-facto open border with Northern Ireland, which is a part of the UK. One of the provisions requires checks on goods arriving from England, Scotland, and Wales, to prevent them from entering the EU's single market. London is expected to scrap most of the checks.

The UK representative Lewis said changes were necessary because the protocol was disrupting the lives of people, hindering government institutions, and "fundamentally undermining" the 1998 Good Friday accord that ended the violence between Irish nationalists and UK loyalists.

Johnson's government has 'refused to engage'

Sinn Fein leader Mary Lou McDonald said the protocol was working, and that London could work with Dublin and the EU instead of making unilateral changes.

She also said that Boris Johnson's cabinet "has refused to engage."

"It has not been constructive, it has sought a destructive path, and is now proposing to introduce legislation that will undoubtedly breach international law," she told Sky News.

McDonald also accused the UK prime minister of using Northern Ireland to boost his position after facing a confidence vote on Monday.

The EU has pledged a strong response if the UK decided to change the trade rules.

The latest escalation comes after Sinn Fein achieved a historic victory in Northern Ireland last month. For the first time ever, the Irish nationalist party controls a plurality of seats in the Northern Ireland Assembly, securing 29% of the votes compared to 21.3% won by their main rivals, the loyalist DUP.

dj/kb (Reuters, AFP)