The European Union threatened on Wednesday to ban exports of COVID-19 vaccines to Britain to safeguard scarce doses for its own citizens facing a third wave of the pandemic that would jeopardize plans to restart travel this summer.

With the number of COVID-related deaths in the EU topping 550,000 and less than a 10th of the bloc's population inoculated, European Commission head Ursula von der Leyen said the epidemiological situation was worsening.

"We are in the crisis of the century," she told reporters.

"We see the crest of a third wave forming in member states, and we know that we need to accelerate the vaccination rates."

Von der Leyen said the flow of vaccine products was smooth with the United States but aired frustration over a lack of deliveries from AstraZeneca in Britain. She said 10 million doses had gone from EU plants to the former member state.

"We are still waiting for doses to come from the UK, von der Leyen said in the latest sign of souring ties between Britain and the 27-nation bloc since Brexit.

"If this situation does not change, we will have to reflect on how to make exports to vaccine-producing countries dependent on their level of openness. We will reflect on whether exports to countries with higher vaccination rates than us are still proportionate."

British Foreign Minister Dominic Raab hit back, accusing the Commission of brinkmanship and calling for an explanation after the government had received assurances that the EU did not intend to curb exports which are under contract.

"I think it takes some explaining because the world's watching ... It also cuts across the direct assurances that we had from the Commission," Raab said. "We expect those assurances and legal, contracted supply to be respected."

"Frankly, I'm surprised we're having this conversation. It is normally what the UK and EU team up with to reject, when other countries with less democratic regimes than our own engage in that kind of brinkmanship."