European Council President Charles Michel accused the U.K. of blocking exports of Covid-19 vaccines—prompting a vehement denial from the British government.

In a newsletter published Tuesday, Michel said he “was shocked” by “accusations of ‘vaccine nationalism’ against the European Union.”

“The facts do not lie,” he wrote. “The U.K. and the U.S. have imposed an outright ban on the export of vaccines or vaccine components produced on their territory.”

U.K. Foreign Secretary Dominic Raab hit back in a letter to Michel, saying the claim is “completely false,” according to a person familiar with the matter. “The U.K. government has not blocked the export of a single Covid-19 vaccine or vaccine components,” Raab wrote.

A representative of the EU’s delegation to the U.K. has been summoned to the Foreign Office to discuss the issue further, the person said.

The spat marks an escalation in tensions 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 program—dogged by delays—has struggled to keep pace with the British. About a third of the U.K. population has received a dose, compared with 6% in the EU, according to data compiled by Bloomberg.

In January, the EU briefly threatened to control the flow of vaccines into Northern Ireland, and thus the U.K., 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.

Michel appeared to row back later Tuesday, writing on Twitter that he was “glad if the U.K. reaction leads to more transparency and increased exports.”

Meanwhile, U.K. Prime Minister Boris Johnson is facing a legal challenge from the bloc after unilaterally announcing he will not introduce export documents on food crossing the Irish Sea to Northern Ireland from April 1, as previously agreed.

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 U.K. that include trade tariffs, an EU official said.

On Monday, Johnson called the tensions “teething problems” which could be “ironed out” with “goodwill and imagination.”