Brussels on Wednesday demanded that tens of millions of British-made coronavirus vaccines be diverted from the UK to make up a supply shortfall in the jabs in the EU.
The European Commission said it was contractually entitled to doses from two UK plants making the AstraZeneca/Oxford University vaccine as its row with the British-Swedish pharmaceutical giant deepened.
The British Government said it had a deal with AstraZeneca to supply 100 million doses of the vaccine with agreed delivery schedules.
British sources said that, once the UK factories had fulfilled their commitment to Britain, AstraZeneca would be free to supply other countries – something the firm's CEO, Pascal Soriot, confirmed in an interview on Tuesday.
But Stella Kyriakides, the EU's health commissioner, said: "We reject the logic of first come, first served. That might work at the neighbourhood butchers, but not on our contracts and not in our advanced purchase agreements."
She said there was " no priority clause" in the EU contract between the four production plants in the agreement, two factories in the EU and the two in the UK.
"In our contract it is not specified that any country or the UK has priority... This needs to be absolutely clear," Ms Kyriakides said.
The European Commission's chief spokesman said: "If UK plants are working better are we expecting UK plants to deliver doses to the EU? Yes we do."
EU officials said AstraZeneca had only offered a quarter of the 100 million doses it had promised for the first quarter of the year, meaning Brussels wants 75 million jabs from the British factories.
Brussels had ordered up to 400 million doses in total and paid €336 million (£296.4 million) in advance to secure the supply. The European Medicines Agency is expected to authorise the AstraZeneca vaccine for use in the EU at the end of this week.
Ms Kyriakides rejected Mr Soriot's claim that AstraZeneca had no contractual obligation to supply the vaccines but only to make "best efforts" to provide the jabs.
She said the EU was losing people to the Covid pandemic "every day", telling reporters at a Brussels press conference: "These are not numbers. They're not statistics. These are persons with families with friends and colleagues that have been affected as well.
"Pharmaceutical companies, vaccine developers, have moral societal and contractual responsibilities which they need to uphold. The view that the company is not obliged to deliver because we signed a best effort agreement is neither correct nor is it acceptable.
"The 27 European union member states are united that AstraZeneca needs to deliver on its commitments in our agreements."
In an interview with European newspapers on Tuesday, Mr Soriot said the company's contract with the UK meant the supply coming from the British supply chain would go to the UK first.
He stressed that AstraZeneca was providing jabs to the EU and UK on a not for profit basis and said: "As soon as we have reached a sufficient number of vaccinations in the UK, we will be able to use that site to help Europe as well.
"But the contract with the UK was signed first and the UK, of course, said: 'You supply us first,' and this is fair enough."
AstraZeneca said in a statement on Wednesday: "As each supply chain has been set up to meet the needs of a specific agreement, the vaccine produced from any supply chain is dedicated to the relevant countries or regions and makes use of local manufacturing wherever possible."
The European Commission said it had asked AstraZeneca for permission to publish the contract after coming under pressure to make the agreement public.
It accused Mr Soriot of revealing confidential information, such as the best-effort clause and the production capacity, in his interview.
Bernd Lange, the chairman of the European Parliament's trade committee, said: "Instead of blaming each other in the media, just make the contract public. Since the vaccine is supposedly non-profit, what's the problem?"
The European ombudsman opened a maladministration investigation into the commission's refusal to grant public access to the contract after a complaint.
AstraZeneca announced a cut in supplies to the EU in the first quarter on Friday, which an EU official told Reuters last week amounted to a 60 per cent reduction to 31 million doses, caused by production issues at a factory in Belgium.
The commission said on Monday it would introduce a mechanism that would force companies producing vaccines in the EU to ask it for permission before exporting jabs.
An EU diplomat told The Telegraph: "Given that, according to several reports, Britain was supplied with AstraZeneca vaccines produced in the EU when its British factories faced production shortfalls a few weeks ago, it would only be logical now to deliver vaccines from Britain to the EU."
AstraZeneca held crunch talks with EU officials and representatives of the 27 member states, which have demanded answers over where its vaccines have been sent.