AstraZeneca must "catch up" with its EU Covid-19 vaccine deliveries before it can export any more doses, European Commission President Ursula von der Leyen said.
Ms von der Leyen was speaking after the first day of the European Council's online summit to explain the reasoning behind a plan that gives the bloc greater powers to stop vaccine exports.
It would essentially mean the EU could limit exports to countries that make vaccines but do not send them to the continent.
The blunt words against the UK-Swedish company threatened to deepen a row with Britain, which is looking to secure AstraZeneca shipments from the EU to fill a hole in its supplies, which is threatening its previously smooth vaccination programme.
"I think it is clear that first of all the company has to catch up, has to honour the contract it has with the European member states before it can engage again in exporting vaccines," Ms von der Leyen said.
The EU is engaged in a months-long dispute with AstraZeneca over a shortage of deliveries and disputes over efficacy and side effects.
AstraZeneca is on course to deliver only 30 million of a contracted 120 million doses of its vaccine to Europe by the end of this month.
With the continent in the grip of a third wave of infections, the EU and its 27 governments have come under pressure for their stuttering vaccine strategy.
The bloc is already lagging behind the US and the UK in its inoculations, and EU governments have announced extensions or tightening of lockdowns as the health crisis continues.
“We could have been much faster if all pharmaceutical companies had fulfilled their contracts,” Ms von der Leyen said.
“AstraZeneca has committed to a lower number of doses than was contracted.”
She was backed by French President Emmanuel Macron, who said it was "the end of naivety".
"I support export control mechanisms put in place by the European Commission," Mr Macron said.
"I support the fact that we must block all exports for as long as some drug companies don't respect their commitments with Europeans."
The export-ban plan was not universally well received before the talks, although leaders made no attempt to overturn it on Thursday.
“We’ve agreed with the commission that if companies don’t fulfil their contracts, export restrictions will of course be more likely than when companies do fulfil their contracts with the EU,” German Chancellor Angela Merkel said.
Earlier, Austrian Chancellor Sebastian Kurz criticised the sharing of shots in the EU, saying that some countries were receiving more than their fair share.
As the leader of a coalition of six countries, Mr Kurz demanded a correction, raising tension.
“When member states have a lot less vaccines available to them than others, then I think this is a big issue for Europe,” he said.
”This could cause damage to the European Union like we haven’t seen in a long time.”
On a more positive note, Dutch Prime Minister Mark Rutte said that London and Brussels could agree on a deal for vaccine sharing by the weekend "or soon after", to avoid the imposition of an EU embargo.
Brussels and London have both laid claim to vaccine stocks in an AstraZeneca factory in the Netherlands.
But Mr Rutte said that if Brussels decided to block any exports from the Halix plant to the UK, his government would enforce it.
The conclusions from the first day of the European Council stressed the need to accelerate and intensify "the production, delivery and deployment of vaccines" remains essential.
"We underline the importance of transparency as well as of the use of export authorisations," the council said.
"We recognise the importance of global value chains and reaffirm that companies must ensure predictability of their vaccine production and respect contractual delivery deadlines."
London has bristled at the European Commission this week toughening rules on vaccine exports to include assessments on how well countries such as Britain are doing in terms of vaccinations, compared to the EU.
A joint statement by the UK government and the commission on Wednesday said both sides were looking for ways to co-operate towards a "win-win" compromise, but no details were given.
Britain has mainly been using AstraZeneca for its vaccine distribution programme, with doses largely coming from the company's plants in the UK.
Most of the EU's vaccine exports to Britain were those developed by Pfizer and BioNTech.
But Britain has announced a sudden AstraZeneca shortfall because millions of doses it had expected from India, which has the world's biggest vaccine-making plant, were delayed while New Delhi vaccinates its own population as cases surge.
While Britain has accused the EU of vaccine "nationalism" over its moves, Ms von der Leyen said the bloc was "the region that exports the most vaccines worldwide".
She said that since the start of December, companies in the EU had exported 77 million doses of Covid vaccines.
An EU official said that more than a quarter of those went to Britain.