Taoiseach Micheal Martin has said that trust has been "eroded" over Boris Johnson's move to override key parts of the Withdrawal Agreement with Brussels.
Martin said the decision by the British Government to alter key elements of the Brexit deal is likely to have an impact on future talks.
The Taoiseach contacted Mr Johnson by phone on Wednesday evening to express his "outright opposition" to the move.
A meeting of the Joint Committee between the UK and EU is being held in London after the British Prime Minister announced proposed legislation to alter key elements of the Brexit deal with Brussels regarding Northern Ireland.
Northern Ireland Secretary Brandon Lewis said on Tuesday that legislation to change the Withdrawal Agreement would go against international law in a "very specific and limited way".
Mr Martin said that 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".
"Obviously the Joint Committee is meeting today and will have that tested in full by the European Union leadership," Mr Martin told RTE Morning Ireland.
"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.
"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 that 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 United Kingdom 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.
