Pro-Brexit parties in Northern Ireland have slammed the EU's bid to maintain a 'mini embassy' in Belfast after the UK's Brexit transition period ends later this year.
The EU says it needs an office in the city to help monitor the UK's implementation of the withdrawal agreement - under which the UK Government agreed to checks on trade between Northern Ireland and Great Britain.
The former EU office in Belfast closed in January. But the row over a new EU presence in the city is fuelling concerns in Brussels about how the UK Withdrawal Agreement will be implemented.
A letter seen by RTE says the UK Government agreed to an EU office in the city in February 2019.
"The UK Government supports the continued presence of EU offices in Edinburgh and Cardiff, alongside London and Belfast, given the longstanding relationship the EU has with all devolved nations," Sir Simon McDonald, permanent secretary at the Foreign Office, wrote.
The revelation is thought to be significant because it illustrates collapsing trust between the UK and EU over whether the UK's Withdrawal Agreement with the EU will be implemented.
The UK Government now says an EU office is not necessary and "would in our view be divisive in political and community terms", according to a letter from minister Penny Mordaunt.
DUP Brexit spokesman Sammy Wilson MP said that the EU plan was unacceptable, and welcomed the UK Government's rejection of its plan for a new office.
"We've always argued that when we leave the EU, we leave as the whole United Kingdom," Mr Wilson said. "The proposed office would be a physical symbol that Northern Ireland was different from the rest of the UK and was still part of the EU structure and sphere of influence.
"We do not see any point in such an EU office, and we're pleased that the UK Government has said they will not give any credence to this proposal."
Ulster Unionist leader Steve Aiken felt the EU was trying to use Northern Ireland as a bargaining chip.
"The need for a permanent EU presence, with customs, veterinary and 'peace process' inspectors, is not recognised. The idea of EU 'perm reps' with extra jurisdiction powers like the EU had in the Balkans is simply not acceptable in the UK - and it is inconceivable that any EU country would accept that on their sovereign territory.
"It is more than regrettable that what should be a simple administrative arrangement is being used as a 'bargaining' chip, with NI in the middle again."
TUV leader Jim Allister hit out at the "arrogant" EU demand.
"It is critical that the UK Government continues to rebuff the aggressive attempt by the EU to secure a permanent presence in Northern Ireland," he said.
"Just as in the NI Protocol it sought to annex Northern Ireland as an EU satellite state, so, now it wishes to assert authority in this part of the United Kingdom through this proposed presence. Brussels' writ must cease to run in every part of the UK and, therefore there can be no EU headquarters here.
"The fact that the EU even seeks such is confirmation of its disrespect for UK sovereignty and belief in its arrogant right to meddle in a foreign state."
Anti-Brexit parties in Northern Ireland have also backed the EU's proposal.
SDLP leader Colum Eastwood told the Irish Times on Sunday that UK Minister Michael Gove wrote to his party, Sinn Fein, Alliance and the Greens saying that such an EU office in Belfast was unnecessary.
Asked about Sir Simon's letter, a Government spokesperson said: "The Government's position is that there's no reason why the commission should require a permanent presence in Belfast to monitor the implementation of the protocol."