EU chief Ursula von der Leyen has threatened to halt exports of AstraZeneca's Covid-19 vaccines if the bloc did not receive its promised deliveries first, escalating a row that has fanned international tensions.
"We have the option of banning a planned export. That's the message to AstraZeneca: you fulfil your contract with Europe first before you start delivering to other countries," Ms Von der Leyen told Germany's Funke media group.
The warning comes as the European Union struggles to speed up its Covid-19 inoculation campaign, just as many member states are facing a third coronavirus wave and renewed curbs on public life.
Ms Von der Leyen said Anglo-Swedish pharma giant AstraZeneca had delivered only 30% of the 90 million vaccine doses it had promised for the first quarter of the year.
The company has blamed production delays at its EU plants, but European officials are furious that AstraZeneca has been able to deliver its UK contract while falling short on the continent.
Ms Von der Leyen had on Wednesday already threatened to invoke emergency powers to block European exports of Covid-19 vaccines to ensure "reciprocity" with other suppliers.
In the interview with German newspapers, the European Commission President reiterated that the EU's contract with AstraZeneca states that vaccines destined for the bloc would be produced in both EU and UK plants.
Read more:
Latest coronavirus stories
"But we haven't received anything from the Brits, although we are delivering to them," she said, adding that the European Commission had sent a "formal letter" to the company to complain.
EU-based manufacturers have shipped 41 million vaccine doses to 33 countries since early February, Ms Von der Leyen said, making the bloc one of the world's biggest export regions for Covid-19 vaccines.
"I can't explain to European citizens why we are exporting millions of vaccine doses to countries that are producing vaccines themselves and aren't sending us anything back," she said.
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, where the inoculation campaign has progressed at a much faster pace.
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.
The EU's export ban mechanism must first be triggered in an individual member state and then be approved by the European Commission before it can be enforced.
The mechanism has so far only been applied once, with Italy blocking the export of a 250,000 dose shipment of AstraZeneca vaccine to Australia, citing "persistent shortage" and "delays in supply".
Not all EU members support export bans, which could upset global supply chains, and countries like Belgium and the Netherlands have urged caution.
The EU's troubled relationship with AstraZeneca was dealt another blow earlier this month when several countries, including Ireland, suspended use of its vaccine over fears it may cause blood clots.
The European Medicines' Agency (EMA) on Thursday however declared the jab "safe and effective" and vaccinations have since resumed in Ireland and some other countries.
