The EU has again criticised the UK's decision not to give the EU ambassador to London full diplomatic status.
A row broke out last week after the Foreign Office insisted Joao Vale de Almeida should not have the same privileges as other diplomats.
The BBC's James Landale said the UK did not to want to set a precedent by treating the EU like a nation state.
But the bloc's High Representative for Foreign Affairs, Josep Borrell, said he "does not accept" the situation.
Peers in the House of Lords also branded the move by the UK government as "vindictive" and "offensive".
Under the Vienna Convention, diplomats stationed around the world are entitled to certain privileges, such as immunity from detention, criminal jurisdiction and taxation.
In 142 other countries around the world, the EU has delegations where its ambassadors are all granted the same status as diplomats representing sovereign nations.
But the UK has decided to go against the grain, with the Foreign, Commonwealth and Development Office wanting to treat the EU delegation only as representatives of an international organisation.
'No good prospects'
EU foreign ministers met in Brussels on Monday and discussed the situation.
Mr Borrell said the UK's first signal since the leaving the EU had not been a "friendly one" and there would be "no good prospects" if things continued this way.
He said the EU was not asking for "something new, or special treatment", adding: "We will not accept that the UK will be the only country in the world that does not accept the delegation of the EU as the equivalent of a diplomatic mission."
However, the high representative said he was "confident we can clear this issue with our friends in London is a satisfactory manner".
The issue has also been debated in the Lords, with peers criticising the government's decision.
Labour former Cabinet minister Lord Reid said it would be seen by the international community as "peevish and vindictive", urging ministers to "reverse this blunder and do the honourable thing".
Tory Baroness Hooper also called the decision "gratuitously offensive" to the EU, specifically Portugal where the new ambassador is from, and the Liberal Democrat Baroness Ludford said the government was "squandering goodwill" and acting in a "petty" fashion.
But Foreign Office minister Lord Ahmad of Wimbledon insisted the government wanted a relationship based on "friendly cooperation" with the EU, and conversations were continuing on long term arrangements.