The EU official tasked with overseeing the post-Brexit deal with the UK has warned that London’s refusal to grant its ambassador full diplomatic status has stoked concern in European capitals.

Maros Sefcovic told the Financial Times that the snub was “very important politically” and meant the UK “would treat the European Union delegation in worse terms than any other country on the planet”.

In a further sign of growing tensions, Mr Sefcovic, a European Commission vice-president, also highlighted EU worries about the potential for the UK to diverge from bloc rules in areas such as labour rights and pesticide regulation.

Mr Sefcovic will be the EU’s chief representative on the new “partnership council” with the UK that will oversee the future-relationship deal that UK prime minister Boris Johnson struck with Brussels at the end of last year.

He said the bloc’s foreign ministers had raised a series of questions during talks on Monday about future EU-UK relations, including the spat over diplomatic rights.

Dominic Raab, Britain’s foreign secretary, has insisted that the EU’s ambassador to London should be seen as representing an “international organisation”, rather than being treated as a national envoy.

London has insisted that the rights and privileges are similar, but the EU has warned that the snub is a provocative irritant to future relations — and makes the UK’s policy unique among the 143 countries where the bloc has missions.

“I’m sure that it’s very important for the UK that their diplomats here are properly treated and have appropriate access, and it’s very important for us . . . to have the same in London,” Mr Sefcovic said.

Since formally leaving the EU last January, the UK has had a diplomatic mission in Brussels with the same status as those of other countries around the world such as the US and Russia.

Mr Sefcovic said the matter was raised by every minister who intervened at the meeting on Brexit.

“We do not want to exaggerate or dramatise the situation but we clearly want our British colleagues and partners to know that this is an issue, which we hope we will solve through the discussions,” he said.

“The UK knows pretty well that we are not just an international organisation,” he added, noting that the bloc represents national governments on a range of subjects.

UK officials said that discussions with the EU were ongoing and that the EU representation would be given the privileges and immunities needed to perform its functions.

Mr Sekcovic said that foreign ministers also pressed him about possible British moves to diverge from EU environmental and labour market standards.

The FT reported earlier this month that the UK was exploring watering down EU limits on working time. UK business minister Kwasi Kwarteng has since confirmed the government is reviewing UK employment law, but insisted it would not lead to a reduction in workers’ rights.

“You already saw the questions coming from the member states concerning labour rights, concerning pesticides,” Mr Sefcovic said. “There is the worry, therefore I think the commission is being called upon to be very vigilant on all these issues and we will.”

The EU’s trade deal with the UK sets out a series of safeguards, insisted upon by Brussels, to protect companies from unfair competition.

The agreement stipulates that neither side will reduce environmental and labour standards in place at the end of last year, when Britain left the single market. There is also a system allowing the EU or UK to introduce tariffs on goods if regulations diverge too much over time.

The commission vice-president, whose UK counterpart has yet to be announced, said it was important that the partnership council now quickly get to work given the volume of issues to be addressed as the new trading relationship beds down.

He expects EU-UK relations in 2021 to be “extremely intense".