The US Food and Drug Administration has said it had told Johnson & Johnson that millions of doses of Covid vaccine produced at a troubled plant can not be used because of possible contamination issues.
In a statement, the FDA said "several" batches of vaccine manufactured at the Emergent BioSolutions facility in the city of Baltimore are not suitable for use.
Each batch is known to correspond to several million doses.
Neither the agency nor J&J revealed the precise number doses, but the New York Times placed the number at 60 million, quoting people familiar with the matter.
"These actions followed an extensive review of records, including the production history of the facility and the testing performed to evaluate the quality of the product," said FDA scientist Peter Marks.
Earlier, a European Union official said the EU does not expect Johnson & Johnson will be able to deliver 55 million Covid-19 vaccine doses it had committed to shipping to the bloc by the end of June.
The EU had previously said that it was confident the US pharmaceutical giant could meet its commitments.
The position has changed after the European drugs regulator said J&J doses sent to Europe from the US factory would not be used out of precaution after a case of contamination.
The potential for a shortfall in J&J vaccine supplies to Ireland had already been flagged last month by Health Minister Stephen Donnelly.
He said, under a worst-case scenario, the supply of the vaccine from J&J this month would be 60,000 doses rather than the 600,000 Ireland had been scheduled to receive by the end of June.
On the issue of the doses which had been contaminated, the European Medicines Agency (EMA) did not say today how many shots were affected.
The EMA said it was aware a batch of the active substance for J&J's Covid-19 vaccine had been contaminated in April with materials for the AstraZeneca vaccine also made at the site in Maryland owned by Emergent Biosolutions, ruining a reported 15 million J&J doses.
The production of J&J's vaccine at the site was halted by US authorities and J&J was put in charge of manufacturing at the plant.
The EMA said that batch was not intended for the European Union and that batches of the vaccine released in the region had not been affected by the cross contamination, based on the information it has.
The US FDA is still deciding whether to allow the factory to reopen.
Sixty million AstraZeneca doses produced there and earmarked to be donated abroad are currently being inspected for quality before they can be shipped.
All of the J&J vaccines distributed and used in the United States so far were made in the Netherlands, not in Baltimore.
On the other hand, the FDA said it was greenlighting two batches of J&J vaccine made at the plant -- that is to say, 10 million doses of the one-shot regimen, a source familiar with the matter said.
Additional J&J batches remain under review.
"Johnson & Johnson has committed to producing safe, high-quality vaccines in order to bring health and hope to people everywhere," said J&J executive vice president Kathy Wengel.