Unilateral changes by the UK to the Northern Ireland Protocol are a breach of international law, Foreign Affairs Minister Simon Coveney says.
Britain's Foreign Secretary Liz Truss presented its Northern Ireland Protocol Bill to the UK parliament on Monday.
If enacted, it would see the removal of the requirement for checks on goods entering Northern Ireland from Britain.
It creates a framework to allow ministers in London to introduce changes in four areas covering customs and agri-food safety checks, regulation, subsidy controls and the role of the European Court of Justice.
Ms Truss claims the UK is "acting within international law".
"We are changing the protocol, we are not getting rid of the protocol. I think that is important to acknowledge.
"We have to take the actions to protect the people across the United Kingdom, we have to take the actions to protect peace and stability in Northern Ireland - and that is exactly what we are doing," she adds.
British Prime Minister Boris Johnson says the plans to alter parts of it are "not a big deal".
While Minister Coveney says British excuses for the bill are not valid.
"We are forced because of UK action to respond to - what we certainly see as - a breach of international law.
"I don't think there's any other way to describe this.
"If you are legislating to set aside elements of an international treaty, which is international law, well then you're breaking international law.
"I presume the British government will have some justification for that, I don't believe that will be valid".
And Minister Coveney says it undermines trust between both sides.
"The risk is by unilaterally acting the way they are now, they potentially risk collapsing the protocol.
"I don't believe the EU can accept the approach that the British government has now taken.
"And of course all of the issues that the protocol was designed to resolve, in terms of managing the disruption of Brexit, now get called into question all over again.
"That is why this is an act of bad faith".
Foreign Affairs Minister @simoncoveney says the legislation from the U.K. is an act of bad faith and an absolute breach of international law which the EU cannot accept #Protocol pic.twitter.com/UqzcRSuGsR
— Seán Defoe (@SeanDefoe) June 13, 2022
Mr Coveney held a 12-minute phone call with Ms Truss earlier.
Senior EU figures have also warned of the damage the changes could cause and have hinted at possible legislative action to prevent them.
An EU source says Brussels will restart legal proceedings against the UK over breaching the Brexit withdrawal treaty of 2020.
Maroš Šefčovič, European Commission vice-president, told Ms Truss on Monday that the decision to unilaterally override the agreement was "damaging to mutual trust and a formula for uncertainty".
While a majority of MLAs at the Stormont Assembly in Northern Ireland have signed a joint letter to Mr Johnson, stating their opposition to proposed legislation to amend the protocol.
Additional reporting: IRN