The British Prime Minister Boris Johnson is reportedly drawing up new legislation that will override the Brexit withdrawal agreement on Northern Ireland.
The move could risk the collapse of talks with the EU over a free trade deal.
The UK Labour Party says it would be an act of '"immense bad faith".
Meanwhile, Mr Johnson says Britain will walk away from negotiations in 38 days time.
As part of the terms of the Northern Ireland Protocol, the region is expected to continue to follow some EU rules after the transition period ends in 2021 to ensure there is no hard border.
Northern Ireland deputy First Minister Michelle O’Neill tweeted that any threat of backtracking on the protocol would be a “treacherous betrayal which would inflict irreversible harm on the all-Ireland economy and the Good Friday Agreement”.
Ms O’Neill stressed the need for the protocol to be fully implemented as soon as possible and to “avoid any border in Ireland”.
Scottish First Minister Nicola Sturgeon said the move would constitute a repudiation by the Government of a treaty “freely negotiated by it” and which was described as “oven ready” by Mr Johnson.
She tweeted this would “significantly increase” the likelihood of a no-deal Brexit, and the “resulting damage to the economy will be entirely Tory inflicted. What charlatans”.
