Theresa May is being warned she faces wrecking her hopes of an EU trade deal unless she dramatically changes her plans over the border in the North.
It is one of the key obstacles preventing negotiations moving onto the next phase of Brexit talks.
Phil Hogan, Ireland’s EU Commissioner for Agriculture, claims the issue could be solved by remaining in the customs union and single market, or allowing Northern Ireland to do so.
"I continue to be amazed at the blind faith that some in London place in theoretical future trade agreements," Mr Hogan said in a statement.
"The best possible FTA with the EU will fall far short of the benefits of being in the single market. This fact is simply not understood in the UK."