Taoiseach (Irish PM) Micheál Martin has said the UK should expect a “firm and strong” response from the EU to the proposed Internal Markets Bill.
Speaking on Sunday, Mr Martin also cautioned against “playing politics” with negotiations.
The proposed bill would go against the Withdrawal Agreement, signed by the UK and EU earlier this year.
Mr Martin also stated categorically there would be “no return of a hard border” on the island of Ireland.
“There’s a very firm and strong view emanating from Brussels in how to manage and deal with this,” said Mr Martin, speaking to Irish broadcaster RTÉ’s Week in Politics programme.
“Whatever ploy or strategic approach is attended for the UK side, will be met with a very measured, firm and strong response from the European Union side,” he said.
He described the way the proposed law had been introduced as “no way to do business”, and that the EU was giving prior warning of plans to introduce Internal Markets Bill.
His comments followed earlier remarks by Irish Minister for Foreign Affairs Simon Coveney, who said said the UK government was behaving in an “extraordinary way” over Brexit.
Despite this, Mr Coveney said a free trade deal was still a possibility.
Speaking to BBC’s Andrew Marr programme, he suggested it would be difficult for trade talks between the two sides to continue if the Internal Markets Bill passes through parliament.
“How then can the EU proceed with these negotiations, and put a new agreement in place, which will be the basis for a new relationship, if existing agreements, which aren’t even a year old, are being legislated against?” he said.
Boris Johnson has said the European Union is threatening to impose a customs border in the Irish Sea, separating Northern Ireland from the rest of the UK.
Mr Coveney rejected the suggestion that the EU’s position on having a customs border between Northern Ireland and Britain had hardened after the agreement was signed, calling this a “completely bogus argument”.
Prime Minister Johnson has said an agreement on trade must be done by 15 October, to be ready in time for the conclusion of the transition period
“In my view it is possible to get a trade agreement, it will probably be a basic, pretty thin trade agreement, but it is possible to do that,” said Mr Coveney.
“We can do a deal that prevents tariffs and quotas being imposed on trade between the UK and the EU and the UK and Ireland in the future.”
On Sunday, Alliance MP Stephen Farry said the UK’s admission it could breach international law was “completely outrageous” and that the proposed legislation could be damaging to Northern Ireland.
During the week, Boris Johnson said part of the reason for the Internal Markets Bill was to protect the the Good Friday Agreement and Northern Ireland peace process.
Mr Farry said the suggestion that the legislation was being put forward for “the benefit of Northern Ireland is completely spurious” and that “we have the protocol for a very genuine reason”.
“There was a need to protect the very particular circumstances here in Northern Ireland, especially around the Good Friday Agreement,” he said.
“It is deeply disconcerting to see how that agreement is being twisted to suit the agenda of the hard Brexiteers.”
Writing in the Sunday Times, Sir John and Mr Blair – former Conservative and Labour prime ministers respectively – said the government’s actions were “irresponsible, wrong in principle and dangerous in practice”
The DUP’s East Antrim MP Sammy Wilson dismissed their claims as “nonsense”, but said his party will table amendments to the Internal Markets Bill.