EU demands that AstraZeneca make up delays of its Covid-19 vaccine by supplying doses from its UK factories on Wednesday risked setting the bloc and Britain on a post-Brexit collision course.

Both the European Union and former member Britain insisted the Anglo-Swedish pharmaceutical company uphold contractual delivery promises to each of them -- even as 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 told a Brussels media conference.

In London, British Prime Minister Boris Johnson's spokesman said: "We expect contracts to be adhered to. AstraZeneca has committed to two million doses a week here in the UK and we do not expect that to change."

The row was triggered last Friday when AstraZeneca informed the EU that it could only supply 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 AstraZeneca vaccine to two others it has already authorised -- from BioNTech/Pfizer and Moderna -- to help reach a goal of inoculating 70 percent of adults in the EU by the end of August.

The anger became incandescent when AstraZeneca CEO Pascal Soriot on Tuesday gave an interview saying his company was prioritising supplies to the UK, which had signed its contract three months before the EU did, and was required only to make a "best effort" to supply the bloc.

Kyriakides said that went against the terms of the contact AstraZeneca signed with the European Commission.

"The view that the company is not obliged to deliver because we signed a 'best effort' agreement is neither correct nor is it acceptable," she said.

"We reject the logic of first-come, first-served. That may work at the neighbourhood butcher's but not in contracts, and not in our advanced purchase agreements."

- Focus on UK plants -

She noted that 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 briefing journalists on condition of anonymity stressed the European Union had allocated 336 million euros ($406 million) to AstraZeneca to permit it to expand production.

Explanations from the company for the delay had varied and the main one, talking about a "yield problem" in one of the EU-based plants, was unsatisfactory, the officials said.

"We are not told what the real problem is," one of the officials said. As AstraZeneca's other plants -- notably in the UK -- were unaffected, "their story is slightly inconsistent".

"The real issue is that we are not having clarity on the path ahead," the official said, adding: "Which plants are they going to use to fulfil the contract?"

- EU-UK tensions -

Should AstraZeneca start diverting vaccine supply from the two UK plants, however, that could jeopardise 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 pace at which it is inoculating its population -- doing so at five times the rate of EU member states collectively.

A sudden slowdown in those doses would be dramatic, especially as Britain has suffered the highest death toll from Covid-19 of any European country and Johnson is counting on the vaccinations to stem deaths

Tensions between the EU and Britain remain high in the wake of Brexit, with British traders and consumers suffering as they cope with higher costs and bureaucracy outside of the European single market.

The EU, meanwhile, plans to grill AstraZeneca further in a meeting with its executives later Wednesday.

There was some confusion, though, over the videoconference, with one EU official saying the company had abruptly pulled out but AstraZeneca saying it would attend.

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