Michel Barnier said there was “no way” the European Union would consider renegotiating the Northern Ireland Protocol.
The EU’s former chief Brexit negotiator said there was “room” to work on working solutions related to concerns about the protocol’s implementation, but said the bloc should not accept a renegotiation.
Mr Barnier also said the UK government had made a “mistake” in its negotiating strategy with the EU by trying to divide member states.
“The protocol is not the problem, it is the solution to the problems created by Brexit,” Barnier told RTÉ’s Prime Time.
“There is room to work on pragmatic and operational solutions to address certain concerns of the British.
“But there is no way, in my opinion, to agree to a renegotiation of this protocol.”
The protocol was agreed by the UK and the EU as a way to maintain a fluid land border on the island of Ireland after Brexit.
The leaders of the four main Unionist parties in Northern Ireland on Tuesday signed a joint statement opposing the protocol.
The declaration is signed by DUP chief Jeffrey Donaldson, UUP chief Doug Beattie, TUV chief Jim Allister and PUP chief Billy Hutchinson.
In response, Taoiseach Micheál Martin said: “The government remains focused, calm and flexible, in solution mode around the protocol and relations between the European Union and the United Kingdom.”
Trade unionists in Northern Ireland have vehemently opposed terms of the Northern Ireland Protocol, which provides for additional checks on goods arriving in the region from the rest of the UK.
Mr Barnier said the EU needs to be “clear and firm” with the UK on the issue, but said there was “some room” to address concerns about its operation.
Mr Barnier, answering questions in his recent book My Secret Brexit Diary, also said the British had made mistakes in their approach to negotiations.
He said: “I think the British made a mistake in their strategy because they tried to divide us or bypass my team to engage directly with the Member States. It was a mistake.
“I was the negotiator who had been the negotiator of the committee, the Member States and the (European) Parliament, and they did not understand this triple mandate that I received.
“Number two, we were united on the European side by unanimity.”
He admitted that one of the options on the table to resolve the Irish border issue was for Ireland to be excluded from the EU’s single market.
However, Mr Barnier said he personally considered this option “impossible”.
He said he told then taoiseach Leo Varadkar in February 2019 that controls to protect the single market “have to be implemented somewhere, whether around the island or on the island. ‘inside it’.
He added: “For me it is impossible to exclude a Member State because of Brexit from the single market.
“The single market is our common asset, including Ireland. This is the reason why we have tried for three years enough to find a solution with the British and finally we have found the solution with the British.
Mr Barnier said solutions had been found first with Theresa May and then again with Boris Johnson – and insisted the UK Prime Minister knew exactly what he was committed to.
He said: “With Boris Johnson. Neither without him, nor against him. With him, with his team, a professional team.
“And Boris Johnson knows exactly what he signed.”
He also revealed that the EU is concerned that other member states will follow the UK out of the bloc.
Boris Johnson knows exactly what he signed
“There is always a risk, but we have to understand the reasons,” he said.
“Very British reasons, social anger, popular sentiment, which is quite different from populism. We have to understand, listen and respond.
“I think the EU is starting to respond with a change in policy. Foreign policy, trade policy, industrial policy, migration policy too.
“We must continue to scale up the change.
“Because Brexit was unlikely, even for Farage and a few others, but it happened. So we have to be very careful. “