Royal Navy medics prepare syringes ahead of giving injections of the Oxford/AstraZeneca Covid-19 vaccine to members of the public at a vaccination centre set up at Bath racecourse in Bath, southwest England on January 27, 2021. (Photo: AFP)

AstraZeneca's two UK plants making Covid-19 vaccine must share production with the EU under the contract the drugs firm signed with Brussels, officials said on Wednesday.

"Are we expecting the UK plants to deliver doses? The answer is yes," one of the officials said, briefing journalists on condition of anonymity.

A second official said AstraZeneca's two UK plants were the priority suppliers for the EU contract, followed by one in Belgium and another in Germany.

"Let's put the sequence there so you don't have doubts. There was no secondary, or I would say backup, role for those two plants," the official said.

The official added that AstraZeneca had announced a delay to dose supply to the EU amounting to just "a quarter" of what was promised.

That was "unacceptable," the official said, pointing to 336 million euros ($406 million) in EU funds that had been allocated to the firm for the vaccine 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 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?"