EU chief negotiator Michel Barnier has urged the Government to create a "close economic partnership" with Brussels as he warned that Northern Ireland would be the part of the UK most impacted by Brexit.
In an address at Queen's University Belfast, he said frictionless trade would be impossible after Brexit but he voiced other concerns about the future.
"I know peace is fragile. It's about people much more than trade or the economy," he said.
When asked by a banker in questions afterwards if Brussels could have avoided Brexit by giving former Prime Minister David Cameron more concessions, Mr Barnier said: "I never work with nostalgia. We cannot rewrite history."
Earlier, the EU's chief negotiator met Sinn Fein deputy First Minister Michelle O'Neill and DUP Economy Minister Diane Dodds - representing Arlene Foster - at Stormont.
Mr Barnier told his audience at Queen's that he understood the economic fears about the future checks and controls for goods entering Northern Ireland from Britain.
"But Brexit unfortunately has consequences that we must manage. The UK has chosen to become a third country, to leave the Single Market and the Customs Union, to leave behind the EU's framework of common rules, common supervision and common Court of Justice," he said.
"It has chosen to create two regulatory spaces. This makes frictionless trade impossible. It makes checks indispensable."
He said the checks had to take place somewhere.
"As the whole point of the protocol is to avoid a hard border and protect the all-island economy, it was clear that they could not take place at the land border between Ireland and Northern Ireland," he said.
"The only real option was to use Northern Ireland's other entry points. This is also where such checks are the easiest to implement. And controls will also take place in Dublin and other EU entry points."
Mr Barnier said negotiating the future UK-EU partnership would be a huge challenge. "A new clock is ticking," he said.
"Eleven months is extremely short. Prime Minister Johnson has said he will not extend this period. This means that the UK will leave the Single Market at the end of this year.
"The EU's ambition is to create a close economic partnership. One that is based on a level playing field. That is the only way we will be able to achieve a truly ambitious deal. A deal that benefits both sides. A deal that - even if it will never match what we have now - lives up to our ambition to remain the best of friends and allies."
Mr Barnier told his audience how on a personal level "for many reasons", he regretted Brexit. "I will always continue to believe that we are better off together than alone - especially in today's world. That is why I will remain a patriot and European," he said.
"But I will also always have profound respect for the UK, its people, culture, great leaders - such as Winston Churchill - and its solidarity with the rest of Europe during its darkest hours.
"That is why at the beginning of this new chapter of European history, I would really like to wish the UK well - speaking here in Northern Ireland - as it embarks on this journey."
Mr Barnier said he was pleased to meet Northern Ireland's political leaders jointly on Brexit as his previous discussions with them had been conducted separately. He welcomed the restoration of devolution.
The DUP and Sinn Fein did not appear together afterwards because of their differences on Brexit, but Mrs O'Neill said Mr Barnier was right to point out the risk of a "cliff edge" come December 31, if a deal was not agreed.
"We are all very clear that we could come towards the end of the year and there could be a cliff edge and we could end up coming out with no trade deal for the future," she said.
"That's going to be catastrophic for trade patterns for the future - for many, many decades. That's why we have the [Ireland/NI protocol in the Withdrawal Agreement]."
Mrs O'Neill said there was less than a year to negotiate the sort of trade deal that was usually conducted over a far longer time frame. "I would be very concerned if it is doable in the 11 months," she said.
Mrs O'Neill and Mrs Foster will meet with members of the UK Government and the devolved administrations in Cardiff today to discuss the UK's relationship with the EU after Brexit. Michael Gove, Chancellor of the Duchy of Lancaster, will also attend.
Mr Gove said that "2020 will be a year of growth and opportunity as we level up across all four corners of the country and strengthen our Union". He added: "This has started positively with the restoration of the Executive in Northern Ireland, and I look forward to seeing First Minister Arlene Foster and deputy First Minister Michelle O'Neill back around the table today for the first time since 2017."