EU demands on Wednesday that AstraZeneca make up delivery delays of its Covid-19 vaccine by supplying from its UK factories risked setting the bloc and Britain on a post-Brexit collision course.
The EU and former member Britain insisted the Anglo-Swedish pharmaceutical company uphold contractual delivery promises to each of them, but the company said there was not enough to go around.
"The 27 European Union member states are united that AstraZeneca needs to deliver on its commitments in our agreements," EU Health Commissioner Stella Kyriakides said in Brussels.
In London, Prime Minister Boris Johnson said he expected AstraZeneca to honour its commitment to deliver two million doses a week to the UK from its plant in north Wales, where a bomb scare paused production for a few hours on Wednesday.
"All I can say is that we're very confident in our supplies, we're very confident in our contracts and we're going ahead on that basis," Mr Johnson said.
The dispute was sparked last Friday when AstraZeneca told the EU it could supply only a quarter of the vaccine doses it had promised for the first three months of this year.
That infuriated the European Commission, which is planning this week to add the vaccine to two others it has already authorised, from BioNTech-Pfizer and Moderna.
They were to help reach a goal of inoculating 70 per cent of EU adults by the end of August.
The anger increased when AstraZeneca chief executive Pascal Soriot on Tuesday said his company was giving priority for its supplies to Britain.
The UK signed its contract three months before the EU did, and was required only to make a "best effort" to supply the bloc.
Ms Kyriakides said this claim was "neither correct nor is it acceptable".
"We reject the logic of first come, first served," she said. "That may work at the neighbourhood butcher's but not in contracts, and not in our advanced purchase agreements."
The tensions eased slightly after Mr Soriot spoke to the EU's vaccines team on Wednesday night, with both sides saying the meeting had been constructive.
"We have committed to even closer co-ordination to jointly chart a path for the delivery of our vaccine over the coming months, as we continue our efforts to bring this vaccine to millions of Europeans at no profit during the pandemic," an AstraZeneca spokesman said.
But Ms Kyriakides complained of a "continued lack of clarity on the delivery schedule".
"The EU remains united and firm contractual obligations must be met," she tweeted.
Earlier on Wednesday Ms Kyriakides said AstraZeneca had four operating vaccine plants in Europe – two in Britain and two in the EU – and the contract made no distinction between them in terms of the contractual volumes to be supplied.
EU officials said the bloc had allocated €336 million ($406m) to AstraZeneca to permit it to expand production.
Explanations from the company for the delay varied and the main one, talking about a "yield problem" in one of the EU plants, was unsatisfactory, the officials said.
"We are not told what the real problem is," one said.
Because AstraZeneca's other plants, notably in the UK, were unaffected, "their story is slightly inconsistent".
If AstraZeneca started diverting vaccines from the two UK plants, that could jeopardise Mr Johnson's commitment to have 15 million Britons vaccinated by mid-February.
Already, thanks mainly to the AstraZeneca vaccine, Britain is one of the leading countries for the speed of its vaccination programme, doing so at five times the rate of EU member states collectively.
A sudden slowdown in those doses would be dramatic, especially because Britain has suffered the highest death toll from Covid-19 of any European country and Mr Johnson is counting on the vaccines to curb deaths.
Tensions between the EU and Britain are high after Brexit, with UK traders and consumers suffering as they cope with higher costs and bureaucracy outside the European single market.