AstraZeneca's reasons for its decision to slow down supplies of COVID-19 vaccines to the EU are "unacceptable", according to the bloc's health commissioner.
According to reports, that meant a cut of 60% to 31 million doses - coming just weeks after Pfizer also announced supply delays.
AstraZeneca is believed to have received an upfront payment of £298m when the 27-nation bloc sealed a supply deal with the company in August for at least 300 million doses.
Under advance purchase deals sealed during the pandemic, the EU makes down-payments to companies to secure doses, with the money expected to be mostly used to expand production capacity.
Stella Kyriakides, the European Commissioner for Health and Food Safety, said in a statement: "This new schedule is not acceptable to the European Union.
"The European Union has pre-financed the development of the vaccine and the production and wants to see the return.
"The European Union wants to know exactly which doses have been produced by AstraZeneca and where exactly so far and if or to whom they have been delivered.
"The answers of the company have not been satisfactory so far... The European Union wants the ordered and pre-financed doses to be delivered as soon as possible. And we want our contract to be fully fulfilled."
The vaccine is expected to be approved for use in the EU on 29 January, with first deliveries expected from 15 February.
An AstraZeneca spokesman said the company was doing everything it could to bring its coronavirus vaccine to millions of Europeans as soon as possible.
Meanwhile, it has been revealed the company, which has its headquarters in the UK, faces wider supply problems.
Earlier on Monday, Australia's health minister Greg Hunt announced AstraZeneca had advised the country it had experienced "a significant supply shock", which would cut supplies in March below what was agreed.
He did not provide figures.
Thailand's health minister Anutin Charnvirakul said the firm would be supplying 150,000 doses instead of the 200,000 planned, and far less than the one million shots the country had initially requested.
The UK is particularly reliant on the Oxford-AstraZeneca jab and it has been key to the country's vaccine rollout so far, but Health Secretary Matt Hancock has warned that supply across the board is "tight".