The European Union will make pharmaceutical companies respect contracts they have signed for the supply of Covid-19 vaccines, according to European Council President Charles Michel.
On Friday, AstraZeneca also said that initial deliveries to the region will fall short because of a production glitch.
"We plan to make the pharmaceutical companies respect the contracts they have signed ... by using the legal means at our disposal," Mr Michel said on Europe 1 radio.
He did not mention possible sanctions but said that the EU would insist on transparency about the reasons for the delays.
He said that after Pfizer's first warnings about delays of several weeks, the EU had managed to reduce these delays by taking a tough stance.
"We banged our fist on the table and finally announced delays of several weeks turned into a slowdown of deliveries,"he said.
Yesterday Italian Prime Minister Giuseppe Conte lashed out at vaccine suppliers, saying delays in deliveries amounted to a serious breach of contractual obligations.
A senior health official has said Italy will have to rethink its whole vaccination programme if supply problems persist, after the country was forced to cut its daily roll-out of Covid-19 shots by more than two thirds.
"This is unacceptable," Mr Conte said in a Facebook post. "Our vaccination plan ... has been drawn up on the basis of contractual pledges freely undertaken by pharmaceutical companies with the European Commission."
Italy says Pfizer deliveries were 29% lower than planned this week and would be down 20% next week.
The head of Italy's higher health council, Franco Locatelli, told a press conference they were expected to return to agreed levels from 1 February.
In the meantime, vaccinations in Italy have slowed to 20,000-25,000 a day from peaks of more than 90,000 around two weeks ago, Mr Locatelli said.
Mr Conte said expected delays in the vaccine by AstraZeneca were even more worrying, adding Italy would receive 3.4 millio ndoses instead of 8 million in the first quarter if the 60% reduction were confirmed.