The EU's chief Brexit negotiator Michel Barnier has said it would be "wise" for the UK to find a solution to the dispute over the UK refusing to grant the EU's ambassador in London full diplomatic status.
In an interview with RTÉ News, he said: "I made this point several times with the British negotiator David Frost in the last few months. I know the temptation of British leaders to speak about the EU as an international organisation. We are much more than an international organisation and the UK knows that fact quite well."
The row surfaced after the leak of a letter from the EU's high representative for foreign policy Josep Borrell to the UK foreign secretary Dominic Raab on the UK's refusal to grant Joao Vale de Almeida full diplomatic status.
The letter, leaked to the BBC, stated: "The arrangements offered do not reflect the specific character of the EU, nor do they respond to the future relationship between the EU and the UK as an important third country.
"It would not grant the customary privileges and immunities for the delegation and its staff. The proposals do not constitute a reasonable basis for reaching an agreement."
In a statement the British Foreign Office said: "The EU, it's Delegation and the staff will receive the privileges and immunities necessary to enable them to carry out their work in the UK effectively."
Earlier, Mr Barnier was awarded the European of the Year by the European Movement here in Ireland.
He told RTÉ News of his experience negotiating both the Withdrawal Agreement and the future relationship treaty over the past four years, including his part in negotiating the Northern Ireland Protocol.
Asked about the clamour from UK business groups for flexibility from the EU on the controls and formalities hitting UK-EU trade as a result of Brexit, Mr Barnier said: "It cannot be, it will not be, business as usual and we worked a lot to reach this agreement on Brexit, economic and trade Brexit.
"It's done and now we have to implement this treaty. It cannot be about any kind of renegotiation."
Mr Barnier said that technical solutions could be found, just as they had been found for the implementation of the Northern Ireland Protocol, but there could be no cherry picking and the EU UK Trade and Cooperation Agreement (TCA) would have to be respected.
"I can understand we need some time to implement precisely and correctly some of the provisions of the agreement. I understand we need a time of adaptation everywhere, for businesses, particularly small businesses, but we cannot work in contradiction of the treaty.
"We found concrete, operational solutions for some key issues [in the Northern Ireland Protocol]. We are ready to do that, but not any kind of contradiction with the treaty or the substance of the treaty."
Mr Barnier said the EU would strictly apply the rules of origin aspects of the treaty, which have hit the movement of goods from the continent to Ireland if they are reboxed or consolidated in UK distribution hubs.
"When you speak about the rules of origin, you are speaking about preserving and defending the jobs in the EU, nothing else.
"Preserving and defending our jobs, particularly the jobs of young people in the EU. This is what's at stake when you speak about rules of origin, that are imported into the UK from everywhere in the world," he said.
"The rules of origin have been negotiated very carefully by the EU with the UK, the UK with the EU. There is no way to go further and change this subject at this point."
Mr Barnier, who will remain as a special advisor to the President of the European Commission Ursula von der Leyen, said the negotiations were so difficult at times, especially when Theresa May failed as prime minister on three occasions to get the Withdrawal Agreement through the House of Commons, that he believed no deal was possible.
"Last year, once or twice [during the trade negotiations] the gap was so large between us - not only on fisheries - that it would have been possible not to reach a deal.
"But President von der Leyen didn't want to close the door, and we never closed the door to the Brits. We have always been ready to continue. Finally, I thought that a trade deal, and it's more than a trade deal, was so important for both sides...that it was a common interest to reach a deal."
He said the solidarity shown to Ireland over the Irish border issue was a reflection of how the EU had sought to carefully protect the interests of all member states in the negotiations.
This included Finland's concerns over aviation, the issue of Gibraltar for Spain, and British military bases in Cyprus.
"From day one the EU27 reaffirmed and reaffirmed every week and every month their solidarity. Meaning that each concern of every member state was taken into account. It was applied for each and every member state, not only Ireland," he told RTÉ News.
Asked if there was a risk that the Northern Ireland Assembly could reject the key elements of the Protocol in four years' time, if its impact remains onerous on Northern Irish businesses and consumers, Mr Barnier said the Protocol, if properly implemented, would mean progress for Northern Ireland and the whole island.
"Everybody knows precisely the content of this protocol," he told RTÉ News. "We have worked lots with the Brits and the authorities of NI...to find concrete operational solutions. To square the circle, of peace, respect of the Good Friday Agreement, the all-island economy, protection of the single market."
He added: "This Protocol is in fact, if it is correctly implemented, progress for Northern Ireland. It will bring progress for all the island. We will see in four years, and I think at that time everybody will have to take their responsibility."
His abiding memory of the long negotiations over the Irish border was holding a private meeting with a women's group near the border. They had wept as they told him they did not want the peace process to go into reverse because of Brexit.
"Speaking of Ireland," he said, "during these negotiations, it was not first about goods, trade, and the economy, but about people, and peace, the men and women on this island."
The full interview with Michel Barnier will be aired in this week's episode of RTÉ's Brexit Republic podcast