The European Union's chief Brexit negotiator said Britain must accept the realities of Brexit and it was the UK's exit from the EU that had caused tensions between London and Brussels over Northern Ireland, not the divorce protocol.
Britain has been seeking changes to the Northern Ireland protocol of the Brexit deal after the European Commission sought briefly to prevent vaccines from moving across the open border between EU-member Ireland and Northern Ireland.
"Both parties must be conscious of their responsibilities in applying fully this protocol. The difficulties on the island of Ireland are caused by Brexit, not by the protocol," Michel Barnier told a European Business Summit event on Thursday.
"The protocol is the solution."
Just under half of British companies that export goods have run into problems caused by the shift in trade terms with the EU since the start of the year, a survey showed on Thursday.
Barnier said it was clear there would be a period of adaptation, but long term it would not be business as usual. "Brexit means Brexit," he said, repeating a mantra coined by former British Prime Minister Theresa May.
Despite a trade deal without tariffs or quotas, businesses face barriers such as new paperwork and veterinary checks.
"Many of these consequences have not been correctly explained, they have been generally underestimated," Barnier said.
Barnier also said that negotiations on Brexit are finished and cannot be reopened. There can though be "technical solutions" for problems like those related to Ireland, as long as the rules were respected.
Barnier added that EU member states' national authorities will be vigilant for possible circumvention of new restrictions by financial services companies.
He said the EU needs further clarification from the UK before making a decision on financial services equivalence, which would grant Britain access to the bloc's financial market.
"I can just repeat that the equivalence decisions are and will remain unilateral of each party and are not subject to negotiation."