Legislation that Britain will unilaterally bring forward on Monday to scrap some of the rules that govern post-Brexit trade with Northern Ireland will not break international law, minister Brandon Lewis said on Sunday.
"The legislation that we will outline tomorrow is within the law; what we are going to do is lawful and it is correct," the Northern Ireland Secretary told Sky News.
When Britain left the EU, Prime Minister Boris Johnson agreed a protocol that effectively left Northern Ireland in the EU's single market and customs union to preserve the open border with Ireland specified in the Good Friday peace agreement.
Any unilateral move by London to override the treaty will inflame a simmering argument with the European Union.
Ireland's Sinn Fein, the nationalist party that won a historic victory in the Northern Ireland Assembly election last month, said on Sunday Britain would "undoubtedly" break the law by imposing unilateral changes to the protocol.
Lewis said however the protocol needed to be changed because it was "fundamentally undermining" the Good Friday agreement.
He said it was disrupting the lives of people in Northern Ireland, was stopping government institutions functioning, and was not respecting the UK's own internal market.
Lewis declined to say how the protocol would be changed, but said the government would set out the legal basis on which it was bringing forward the legislation.
Sinn Fein president Mary Lou McDonald said London could work with Dublin and Brussels to improve the application of the protocol.
"There is a willingness here, there is a willingness to engage by the European Commission, but the British government has refused to engage," she told Sky News from Dublin.
"It has not been constructive, it has sought a destructive path, and is now proposing to introduce legislation that will undoubtedly breach international law."