The European Commission said on Monday it had launched legal action against AstraZeneca for not respecting its contract for the supply of COVID-19 vaccines and for not having a "reliable" plan to ensure timely deliveries.

AstraZeneca said in response that the legal action by the EU was without merit and pledged to defend itself strongly in court.

"AstraZeneca has fully complied with the Advance Purchase Agreement with the European Commission and will strongly defend itself in court. We believe any litigation is without merit and we welcome this opportunity to resolve this dispute as soon as possible," AstraZeneca said in a statement on Monday.

Under the contract, the Anglo-Swedish company had committed to making its "best reasonable efforts" to deliver 180 million vaccine doses to the EU in the second quarter of this year, for a total of 300 million in the period from December to June.

But AstraZeneca said in a statement on March 12 it would aim to deliver only one-third of that by the end of June, of which about 70 million in the second quarter. A week after that, the Commission sent a legal letter to the company in the first step of a formal procedure to resolve disputes.

"The Commission has started last Friday a legal action against AstraZeneca," the EU spokesman told a news conference, noting all 27 EU states backed the move.

"Some terms of the contract have not been respected and the company has not been in a position to come up with a reliable strategy to ensure timely delivery of doses," the spokesman said, explaining what triggered the move.

Under the contract, the case will need to be resolved by Belgian courts.

"We want to make sure there is a speedy delivery of a sufficient number of doses that European citizens are entitled to and which have been promised on the basis of the contract," the spokesman said.

"Every vaccine dose counts," EU health commissioner Stella Kyriakides said on Twitter, announcing the legal proceedings against AstraZeneca.

Germany, France and Hungary were among EU states that were initially reticent to sue the company, diplomats said, but eventually they supported the move.

The EU wants AstraZeneca to deliver the promised 300 million doses, but in a further sign of its irritation towards the company, it has already forgone another 100 million shots that it had an option to buy under the contract signed in August.