The EU on Wednesday threatened to invoke emergency powers to block European exports of Covid-19 vaccines to ensure "reciprocity" with other suppliers, urging Britain to send Europe more jabs.
EU chief Ursula von der Leyen gave her warning as Brussels unveiled its plan for a vaccine travel certificate, part of its effort to free up travel despite an emerging third wave of the pandemic.
Europe's vaccination campaign has struggled to get off the ground due to delayed deliveries and a bitter row with pharma giant AstraZeneca and fears over the safety of its vaccine.
The EU has already set up special oversight of vaccine exports in which manufacturers contracted to supply Europe must declare if they intend to export doses outside the bloc.
Most of the EU's worry is over Britain, home of the AstraZeneca vaccine, where the vaccination campaign has progressed at a much faster pace than in the EU.
Brussels has accused London of operating a de facto export ban to achieve its vaccine success, a claim furiously denied by Prime Minister Boris Johnson's government.
But Von der Leyen on Wednesday said the EU was "still waiting" for its AstraZeneca orders to come out of "two sites in the UK", despite the fact that 10 million doses from other manufacturers had entered the UK from the EU.
"This is an invitation to show us that there are also doses from the UK coming to the European Union, so that we have reciprocity," she said.
- 'Crisis of the century' -
The former German defence minister said that "all options are on the table" to resolve the matter and that the vaccine situation would be addressed among EU leaders at talks next week.
"We are in the crisis of the century. And I'm not ruling out any anything for now," she said, briefly mentioning emergency powers used during the OPEC oil shock of the 1970s.
The EU's announced travel certificate, meanwhile, is intended to help restore freedom of movement within the bloc for citizens inoculated against the coronavirus.
The certificate will show "whether the person has either been vaccinated, or has a recent negative test, or has recovered from Covid, and thus has antibodies," von der Leyen said.
The idea is to allow inoculated tourists to get around restrictions on non-essential travel that have spread across Europe, as a second and third wave of Covid-19 infections brought much intra-EU travel to a standstill.
"With this digital certificate we aim to help member states reinstate the freedom of movement in a safe, responsible and trusted manner."
The plan, however, will face stiff resistance from many member states, a key concern being that those still awaiting vaccinations would be discriminated against.
In addition, some member states are worried that the legal path to create the pass, which would include approval by European Parliament, will take too long, with the summer holidays just three months away.
The commission is adamant that the process can be fast-tracked and is working to have it ready by June.