The European Union has called on the British Government to grant the EU's first ever ambassador to the country full diplomatic status.
In a post-Brexit spat, British Prime Minister Boris Johnson's Government is refusing to grant full diplomatic status to Joao Vale de Almeida, the 27-nation EU's envoy to the UK.
The government says the EU is an organisation rather than a country.
Mr Vale de Almeida would be entitled to immunity from taxation and prosecution, among other rights accorded to ambassadors, under the Vienna Convention.
EU foreign policy chief Josep Borrell said: "We will not accept that the United Kingdom will be the only country in the world that doesn't recognise the delegation of the European Union (as) the equivalent of a diplomatic mission."
Mr Borrell said that EU delegations are recognised by 143 countries "without a single exception".
Almost exactly a year after Brexit, he said: "It's not a friendly signal; the first one the United Kingdom has sent to us immediately after leaving the European Union. If things have to continue like this there's no good prospects.
"We do not ask for something new, or we don't ask for special treatment."
Mr Borrell gave no indication of what the EU might do if the problem is not resolved soon.
The Foreign Office said last week that "the EU, its delegation and staff will receive the privileges and immunities necessary to enable them to carry out their work in the UK effectively".
