EU Council chief Charles Michel on Sunday warned the manufacturers of the Covid-19 vaccines of possible legal consequences after they announced delays in delivering the doses and smaller deliveries due to supply chain issues.
"We expect that the contracts confirmed by the pharmaceutical firms will be fulfilled," Michel told French broadcaster Europe 1.
Michel warned the European Union could also use "legal means" to ensure the contractual obligations were met.
After the manufacturing partners BioNTech and Pfizer announced delays in deliveries last week, the British-Swedish firm AstraZeneca also said it would initially deliver fewer doses to the EU than agreed.
Although it was understandable there were problems, these needed to be understood fully, Michel said.
He also acknowledged it would be difficult to meet the EU's goal to vaccinate 70 per cent of its adult population by summer.
Italy's Prime Minister Giuseppe Conte also sharply criticized the delays in a Facebook post late on Saturday.
"These delays constitute grave contractual violations that are causing huge damage in Italy and other European countries," Conte wrote.
"If the reduction of 60 per cent of the deliveries in the first trimester is confirmed, this would mean that Italy receives 3.4 million doses instead of 8 million," Conte wrote.
Conte said Health Minister Roberto Speranza had already spoken with AstraZeneca representatives and announced he would take legal steps against the firm, just like against Pfizer-BioNTech, to up the pressure regarding the sales contracts.
AstraZeneca developed the vaccine together with Britain's Oxford University, and the vaccine is already in use in Britain.
It is expected to be approved by the EU next week.
Italy has vaccinated 1.34 million of a population of 60 million.