An adviser to Jean-Claude Juncker has poured cold water on the Foreign Affairs Minister’s proposals to have no border between Ireland and the North.
Simon Coveney suggested last week that it might be possible for the North to maintain a relationship with the customs union, to allow for the free movement of trade.
Details of the UK Government's plan for Europeans living in the UK after Brexit will be published later.
Last week, Foreign Affairs Minister Simon Coveney outlined his vision for a borderless Ireland, if the UK does leave the Customs Union, as planned.
"It's not so much about a soft or hard border, it's about an invisible border effectively
"To achieve that we need to draw up a political solution here which doesn't have a precedent in Europe.
"It would mean that the North would have links to the customs union, while the UK would be outside it."
But Catherine Day, special adviser to Commission President Jean-Claude Juncker says it would be too complicated;
"Say the British leave the customs union and in 15-years time they o this less attractive trade deal with any other part of the world.
"Which rule would apply in Northern Ireland, the EU rules, or the British rules?
"I find it hard to imagine how the UK could cut better trade deals with international partners than the EU."
Ms Day, who spent ten years as Secretary General to the EU Commission, says Britain is deluded if it thinks it won’t have to make compromises;
"Their negotiating position seems to be 'we want everything we like, and we don't want anything we don't like', and you cannot send people into technical negotiations with that as a mandate."
Theresa May's to give further details about the British government's position on rights for EU citizens living in the UK after Brexit.
Axel Antoni's a spokesperson for '3 million' - a group representing EU citizens there.
He says the proposals so far have been too vague:
"We called it pathetic because it was not really what we wanted.
"The two benchmarks we have are the vote leave promise; no EU citizen will be treated less favourably than current, and the proposal on Friday didn't match that.
"The other benchmark is the really generous offer that has been put on the table by the EU to protect the British in Europe."