It cannot be business as usual for traders on both sides of the border after Brexit, the European Union's chief negotiator warned on a trip to Londonderry yesterday.
But Michel Barnier also told members of the business community that Northern Ireland will continue to benefit from European Peace Programme funds after the UK leaves the EU.
Mr Barnier was greeted in the Guildhall by Sinn Fein MEP Martina Anderson, who suggested that the DUP's Diane Dodds and Ulster Unionist Jim Nicholson were failing to represent the unionist people by "snubbing" Mr Barnier.
Ms Anderson accused the two unionist MEPs of "bad manners".
Mr Barnier, who enjoyed a walk on the Peace Bridge and Derry's Walls, also said that while the EU is open to any solution the UK Government has to maintain the Good Friday Agreement and an all-Ireland economy, its preferred solution was the so-called backstop.
This means if no better solution is found Northern Ireland would continue to follow EU rules relating to the all-Ireland economy and north-south co-operation.
"The UK has decided to leave the union - it is its decision, not our decision," he said.
"I think we must maintain co-operation between Northern Ireland.
"This week the European Commission will propose the new framework for the future financial perspective from 2021 to 2027 and I can tell you we are ready to maintain the peace programme.
"Prime Minister May accepted in December and in March to recognise that we are together to maintain an all-Ireland economy, the peace process, the Belfast Agreement in all its dimensions and it is a line we have to follow together now.
"We agreed on a political framework at this point and now we have to find together the operational and practical solution together.
"We have proposed one solution, the backstop, in our protocol.
"We are open to any solution to maintain the Belfast Agreement in all its dimensions, the all-Ireland economy also to respect the integrity of the single market.
"We have proposed one solution which is operational. I am ready to explain this solution to everybody."
Londonderry Chamber of Commerce president Jennifer McKeever described the private meeting with Mr Barnier as "sobering".
"His exact words were 'it cannot be business as usual' and that Brexit is a 'lose-lose' situation for everyone," she said.
"He was very clear that things have to change and put paid to any illusions people may have that there will no change for us due to Brexit."
Ms Anderson was the only Northern Ireland MEP to meet Mr Barnier.
Last week, Mrs Dodds and Mr Nicholson issued a joint statement welcoming Mr Barnier's visit but expressed disappointment that it was announced by a Sinn Fein MP.
Mrs Dodds said she had already committed to meetings in London and could not meet him.
Yesterday, Ms Anderson said it was a mistake by unionist politicians not to come to Derry.
She added: "I think it is political bad manners that the MEPs and the two council leads from the DUP and the UUP were both invited here and neither of them have turned up.
"I don't think it is a good signal to send out, I don't think it is representative of the Protestant, unionist, loyalist people who would want their own representatives to take the opportunity to express their views."
She added: "Whilst the DUP and UUP are telling us they don't want a hard border in Ireland, another trend of thought is that particularly the DUP who didn't vote for the Good Friday Agreement will not be put out if there is a hard border.
"For some of them, particularly their MPs, they may feel it is a job done."
DUP Foyle MLA Gary Middleton said he would have met Mr Barnier if he had been given the opportunity.
"An invite came in for councillors late on Monday evening which to me seemed like a last minute attempt to bring some credibility to invite unionists, but it was too late notice," he said.
"As the only unionist MLA it would have been a courtesy to send an invite. I would have attended and I could have attended and put across another point of view.
"I felt there would have been an opportunity for Mr Barnier to hear the views of unionist minority within a border community."
Mr Middleton said that while Britain would ultimately leave the EU, Northern Ireland had to remain an integral part of the UK and should have no borders with the mainland.
He added: "The onus is on him to ensure there is no hard border.
"It feels like the EU is out to punish the United Kingdom for leaving rather than working positively for solutions that will benefit us all."