Flexibility by the European Union on the Northern Ireland Protocol has not been reciprocated by the British government, the Tánaiste has said.
Leo Varadkar told a Co-operation Ireland business dinner in Dublin that this is "breeding mistrust in EU capitals."
"The fact that the UK government is talking openly about breaching international law is a matter of concern," Mr Varadkar said.
He said the existing protocol was "co-designed by the UK and the EU/Ireland" and is working.
Mr Varadkar also said that "around 60%" of the MLAs elected to the Northern Ireland Assembly earlier this month do not want the protocol scrapped.
The Tánaiste said that "any British government that claims to be 'pro-union' and any British Prime Minister who is also the Minister for the Union must understand the consequences of imposing on Northern Ireland a policy that is not supported by the majority of the people there".
Earlier the Minister for Foreign Affairs said he "deeply regrets" the "unhelpful" decision by the British government to take unilateral action over the Northern Ireland Protocol.
"Such unilateral action in respect of an internationally binding agreement is damaging to trust and will serve only to make it more challenging to find solutions to the genuine concerns that people in Northern Ireland have about how the protocol is being implemented," Minister Simon Coveney has said.
He added: "I welcome that [Liz Truss] has expressed her preference for a negotiated solution with the EU. The EU has indicated that it is ready and willing to resume talks with the UK at any time."
Speaking at Cathal Brugha Barracks in Dublin, Mr Coveney said there had been "no serious engagement since February" between the EU and British negotiating teams "to find middle ground positions and landing zones that can solve the problems (with the protocol) that are still there".
"Instead of engaging, the British government has decided that it is going to threaten domestic legislation to set aside elements of the protocol which is part of an international treaty... I think that is going to cause a lot more problems than it solves if they go ahead and do that," Mr Coveney said.
The Taoiseach has told the Dáil that the unilateral move by the UK government "is not welcome", "is regrettable" and "does not work".
EU will react with 'all measures at its disposal'
The European Union will respond to unilateral action by the UK over the Northern Ireland Protocol with "all measures at its disposal", according to European Commission Vice-President Maroš Šefčovič.
He said today's announcement by the UK government raised "significant concerns".
Mr Šefčovič listed these concerns in a statement.
"First, because the protocol is the solution agreed between the EU and the UK to address the challenges posed by the UK's withdrawal from the EU for the island of Ireland, and to protect the hard-earned gains of the peace process.
"Second, because the protocol is an international agreement signed by the EU and the UK. Unilateral actions contradicting an international agreement are not acceptable.
"Third, because the Withdrawal Agreement and its protocol are the necessary foundation for the Trade and Cooperation Agreement, which the EU and the UK have agreed upon to organise their overall relationship after the UK's withdrawal."
Mr Šefčovič added: "Should the UK decide to move ahead with a bill disapplying constitutive elements of the protocol as announced today by the UK government, the EU will need to respond with all measures at its disposal."
Maros Šefčovič described the protocol as an "integral part of the Withdrawal Agreement. It avoids a hard border on the island of Ireland, protects the 1998 Good Friday (Belfast) Agreement in all its dimensions, and ensures the integrity of the EU Single Market".
He said the European Commission "stands ready to continue discussions with the UK government to identify joint solutions within the framework of the protocol that would benefit people and businesses in Northern Ireland".
Mr Šefčovič said its "overarching objective is to find joint solutions within the framework of the protocol. That is the way to ensure legal certainty and predictability for people and businesses in Northern Ireland."
Sinn Féin party leader Mary Lou McDonald said in a post on Twitter: "British government announces its intention to legislate to break the law.
"The stuff of a rogue state. Meanwhile government and progress blocked in the North of Ireland."
Alliance Party MLA Sorcha Eastwood said "unilaterally" is "not the way to go" about engendering trust in terms of relationships between the UK and EU.
She told RTÉ's News at One that several elements of Liz Truss’s statement were "contradictory in nature".
There are outstanding issues, such as pet medicine and pet travel, but that those can be worked through and dealt with, she added.
Ms Eastwood said they are still looking at the details in the statement, but that ultimately it is "a lot of rehashed ideas that have been about for a while".
"At the end of the day, all parties have a mandate and the vast majority of MLAs elected to the new Assembly are willing and ready to go to work."