European Council President Charles Michel accused the UK of blocking exports of Covid-19 vaccines, prompting a vehement denial from the British government.
Mr Michel said he “was shocked” by “accusations of ‘vaccine nationalism’ against the European Union".
“The facts do not lie,” he wrote in a newsletter published on Tuesday.
“The UK and the US have imposed an outright ban on the export of vaccines or vaccine components produced on their territory.”
UK Foreign Secretary Dominic Raab hit back in a letter to Mr Michel, saying the claim was “completely false", a source said.
“The UK government has not blocked the export of a single Covid-19 vaccine or vaccine components,” Mr Raab wrote.
A representative of the EU delegation to the UK has been summoned to the Foreign Office to discuss the issue further, the source said.
The dispute is an escalation in tension between the two sides since Britain completed its departure from the bloc in December.
Vaccines have become a flashpoint in the relationship as the EU’s programme, dogged by delays, has struggled to keep pace with the British.
About a third of the UK population has received a vaccination, compared with 6 per cent in the EU, Bloomberg data shows.
In January, the EU briefly threatened to control the flow of vaccines into Northern Ireland, and thus the UK, by using an emergency clause in the Brexit agreement.
In doing so, the bloc reopened one of the most sensitive disputes in the entire divorce: how to avoid the return of full Customs checks on the island of Ireland.
Meanwhile, UK Prime Minister Boris Johnson is facing a legal challenge from the EU after announcing that he would not introduce export documents on food crossing the Irish Sea to Northern Ireland from April 1, as previously agreed to.
European Commission Vice President Maros Sefcovic outlined details of the action to EU ambassadors meeting in Brussels on Tuesday.
Measures could include imposing penalties on the UK, including trade tariffs, an EU official said.
On Monday, Mr Johnson called the tension “teething problems” that could be resolved with “good-will and imagination".