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.
During the week, Boris Johnson said part of the reason for the Internal Markets Bill was to protect the Good Friday Agreement and Northern Ireland peace process.
"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 intended 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".
Speaking earlier in the week, Mr Martin said he was not optimistic of a Brexit deal in light of the UK proposal to override parts of the Withdrawal Agreement.
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 at the end of this year.
"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.
'Acting in bad faith'
On Sunday there was further local and international reaction.
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.
Sinn Féin MLA Caoimhe Archibald said the British government was "acting in bad faith and shows clear intent to disregard the protocol in the withdrawal agreement".
The EU's chief negotiator, Michel Barnier, said the EU could not have been "clearer" when the two sides agreed the Brexit withdrawal agreement last year what the implications would be for Northern Ireland and the rest of the UK.
Responding to Mr Barnier's comments, the UK's chief Brexit negotiator, David Frost, tweeted: "On the Protocol, we indeed negotiated a careful balance in order to preserve peace and the Belfast (Good Friday) Agreement.
"It is precisely to ensure this balance can be preserved in all circumstances that the government needs powers in reserve to avoid it being disrupted."
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.
"The Internal Market Bill as published is not the finished product but it is a massive step forward for business in Northern Ireland," he added.
Ulster Unionist leader Steve Aiken described the Irish government and EU concerns as "self-serving hypocrisy".
He said they were "content to raise the spectre of a land border as anti-Belfast Agreement, whilst at the same time ignoring the anti-agreement reality of an Irish Sea Border".