TAOISEACH Micheál Martin has said that trust has been "eroded" over Boris Johnson's move to override key parts of the EU Withdrawal Agreement with Brussels.
He said the decision by the British government to alter key elements of the Brexit deal, including the Northern Ireland Protocol which is designed to guarantee there will be no hard border on the island, is likely to have an impact on future talks.
Mr Martin contacted the prime minister by phone on Wednesday evening to express his "outright opposition" to the move.
Secretary of state Brandon Lewis admitted in the House of Commons on Tuesday that legislation to change the agreement would go against international law in a "very specific and limited way".
Northern Ireland's top judge has also criticised the British government’s latest Brexit plan.
Lord Chief Justice Declan Morgan told BBC NI's The View that it was "something that may well undermine trust in the government and certainly might undermine trust in the system of the administration of justice".
Mr Martin said his British counterpart assured him that the UK is "fully committed" to meeting its obligations in relation to the protection of the single market.
However, Mr Martin said the legislation published on Wednesday "runs counter to that".
"I made it very clear to him in no uncertain terms our outright opposition to the decision that he and his government took yesterday, and the unilateral nature of the British government's decision to break an international treaty," he told RTÉ Morning Ireland.
"I pointed out very strongly to him that this was very unsettling for Northern Ireland.
"It was dragging Northern Ireland back into the centre stage, that it was bad Northern Ireland politics and would be divisive .
"But more fundamentally, I made a point to him that we all have obligations as political leaders to protect our people from the worst effect of a no-deal and that this intervention was very, very serious and has raised a fundamental issue of trust between the European Union negotiators and United Kingdom and ourselves."
He said the international agreement has been "undermined", which has implications for the conduct of negotiations into the future.
Mr Martin added: "Britain signed up for this because Britain has said consistently that it wants access to the European market for its goods and services, and to maintain the jobs that they have in the UK in all range of companies and sectors, and that's why this particular mechanism was put in in the first place."
He said that the mechanism is there to enable good conduct of business between Ireland and Great Britain.
Meanwhile, the Northern Ireland Affairs Committee has launched a wide-ranging inquiry that will examine the NI Protocol and all its implications on public services, trade, the economy and citizens’ rights in the north.
The inquiry will "anticipate and react to developments in the UK-EU trade negotiations, UK politics, and in the implementation of the Protocol".
