Sinn Féin leader Mary Lou McDonald has said the consequences would be "extremely serious" for Ireland and Britain if Prime Minister Boris Johnson proceeded to disapply sections of the Northern Ireland Protocol through legislation, which is expected next week.
She said such a course of action would warrant a firm response from the European Union and the United States.
Ms McDonald was speaking ahead of a meeting with the European Commission vice president Maroš Šefčovič in Brussels, alongside Sinn Féin vice-president Michelle O'Neill.
"If Boris Johnson persists, and if he insists on disapplying the Protocol or sections of it, that calls into question the Withdrawal Agreement in its entirety, in my view, and the consequences of that could be extremely serious for Ireland, and extremely serious, by the way, for Britain," she said.
"Unilateral action or the threat of unilateral action of that sort, of course, warrants an international response.
"It warrants a firm response from the European Commission and from the United States of America."
Ms McDonald added: "I can only hope that Boris Johnson is listening, that [Foreign Secretary] Liz Truss is listening and they have heard the very, very serious reaction to even the proposal of that.
"We need them now to step up, to honor their word and to act in good faith.
"That's what we're asking for, bearing in mind that the resolution of these issues around the protocol is in the gift of the British government and the European Commission, not the Assembly, not the Executive in the North.
"So why should people who went out 28 days ago and cast their vote in good faith? Why should they be punished because Boris Johnson cannot carry himself as a good faith negotiator?"
Describing the expected legislation as "extremely dangerous" Ms McDonald said the protocol was a necessary consequence of Brexit.
She said the European Commission had been flexible in finding solutions to resolve the issues around the Protocol
The UK is expected to table legislation next week which would disapply parts of the protocol.
Ms O'Neill said the democratic outcome of the election in Northern Ireland is yet to be respected and that the protocol in Northern Ireland is working.
It is not the desires of the people of Ireland, she said, who want to see an agreed way forward.
Speaking on RTÉ's News at One, Ms O'Neill said there will always be issues that arise of the new trading reality, but said there is a forum in which to deal with these issues and accused the British government of using the people of Northern Ireland in its battle with the EU.
"We're missing out. The political uncertainty, the economic uncertainty is all brought onto us," she said.
The problem that we have, she said, is that the British government is not a trusted negotiating partner.
Every time they achieve some flexibility, they pocket it and move on and move the goalposts, Ms O'Neill said.
Meanwhile, Taoiseach Micheál Martin warned the UK that "unilateral action is wrong and not the correct approach" and "would be a serious breach of the Withdrawal Agreement".
Mr Martin also said it was "undermining stability in Northern Ireland".
Speaking during Leaders' Questions in the Dáil, he said that he believes that with "intensified engagement and political commitment the EU and UK process can deliver outcomes responding to the genuine concerns of people in Northern Ireland."
