Boris Johnson’s Government has become embroiled in a diplomatic spat with the European Union over the status of the bloc’s ambassador.
Joao Vale de Almeida has not been given the same status as ambassadors sent by national governments.
Senior Tories questioned the Government’s approach, with one ex-minister warning it could set a “bad precedent” that nations with poor human rights records might follow in an attempt to silence EU diplomats.
Mr de Almeida is the EU’s first ambassador in London after Brexit meant the UK’s departure from the bloc.
Michel Barnier, who led the EU’s negotiating team during Brexit talks, said the UK should be “very careful” about its approach.
“I know the spin of the UK authorities, speaking about the EU like international organisations, but we are not an international organisation,” said Mr Barnier, who is now an adviser to European Commission president Ursula von der Leyen.
“I hope that we will be able to find a clever and objective solution to the status of the EU in London.
“It would be wise in my view to for the UK to find a clever solution.”
Senior Tory Tobias Ellwood described the row as a “silly spat” and insisted the UK should be “better than this”.
Mr Ellwood, chairman of the Commons Defence Committee, said: “This is simply petty.
“(Joe) Biden commits to strengthening alliances and we engage in silly spats which will not help strengthen security and trade cooperation.
“We are better than this.”
Former Europe minister Sir David Lidington said he hoped the Foreign Office “doesn’t pick a fight on this”, warning that non-recognition could set a “bad precedent” for regimes that hate EU ambassadors speaking up in support of human rights.
Tory peer Lord Barwell, who was Mrs May’s chief of staff, said the move was “difficult to understand” and “the UK treating EU diplomats differently than every other independent sovereign state does suggests there is something different about our relationship with the EU”.
“Strange position for Brexiteers to take,” he added.
A Foreign Office spokesman said: “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.”
But Brussels said the EU’s 143 delegations and staff in other parts of the world had been accorded a status equivalent to countries’ embassies under the Vienna Convention, which governs the rules of international diplomacy.
The Vienna Convention grants diplomats immunity from detention, criminal jurisdiction and taxation.
But Whitehall sources insisted that international organisations were offered “very similar privileges and immunities” to diplomatic missions sent by foreign governments.
The UK is continuing to negotiate with the EU over the long-term arrangements for the delegation.
European Commission spokesman Peter Stano said the UK, which, as a member of the EU, was a signature to the Lisbon Treaty which established the European External Action Service diplomatic network, was “well aware of the EU’s status in external relations”.
“Nothing has changed since the UK’s exit from the European Union to justify any change in stance on the UK’s part,” he said.
“The EU’s status in external relations and its subsequent diplomatic status is amply recognised by countries and international organisations around the world, and we expect the United Kingdom to treat the EU Delegation accordingly and without delay.”
He said granting reciprocal treatment based on the Vienna Convention on Diplomatic Relations is “standard practice” between equal partners and we are “confident that we can clear this issue with our friends in London in a satisfactory manner”.
Mr Stano added: “The European Union has 143 Delegations, equivalent to diplomatic missions, around the world.
“Without exception, all host states have accepted to grant these delegations and their staff a status equivalent to that of diplomatic missions of states under the Vienna Convention on diplomatic relations, and the UK is well aware of this fact.”
The BBC reported that the EU’s foreign affairs chief Josep Borrell has written to Foreign Secretary Dominic Raab directly to raise his concerns about the situation.
The dispute with Brussels came as the Foreign Office announced Lindsay Croisdale-Appleby would be the new head of the UK’s mission to the EU, replacing Sir Tim Barrow as ambassador.