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The European Union said Monday it is suing AstraZeneca over an alleged breach of its vaccine supply contract, a dramatic escalation of a months-long dispute over delivery delays that hampered the rollout of shots across the continent.

The 27 nations of the European Union had ordered 300 million doses of the Covid-19 vaccine from the British-Swedish drugmaker to be delivered by the end of June, with an option to purchase an additional 100 million. But deliveries of the vaccine repeatedly fell short, sparking a bitter public fight over the terms of the contract.

AstraZeneca said in March it was aiming to send 100 million doses in total to the union in the first half of the year, just a third of what was expected.

“This action is due to the continuous breach of the terms of the contract and to the lack of a reliable strategy by the company to ensure the timely supply of vaccines in the current circumstances,” Stefan De Keersmaecker, a spokesperson for the European Commission, said Monday.

The European Commission filed its lawsuit in Brussels on Friday, according to De Keersmaecker. An initial hearing will take place on Wednesday, he added. The bloc said last week it would not exercise its option to purchase the 100 million additional doses.

AstraZeneca in a statement denied breaching its contract and said that it “regrets” the European Commission’s decision to launch legal action.

“Following an unprecedented year of scientific discovery, very complex negotiations, and manufacturing challenges, our company is about to deliver almost 50 [million] doses to European countries by the end of April, in line with our forecast,” it said.

“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,” it added.

The lawsuit could mark the start of a lengthy court battle.

The full, unredacted contract between the European Commission and AstraZeneca — first published by Italian broadcaster RAI — includes a clause that appears to release the company from legal action for delays in delivery.

But European Commission officials had previously said they believed the company was acting against the spirit of the contract, pointing to AstraZeneca’s successful delivering to the United Kingdom while failing to meet its targets for the bloc.

“What matters to us in this case, is that we want to make sure that 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,” said De Keersmaecker, adding all 27 countries supported this action.

The lawsuit is the latest in a string of problems for AstraZeneca. The company came under scrutiny over the way it presented its clinical trial data in Europe in the earlier stages of the pandemic, and more recently in the United States.

Decisions from drug regulators in several European nations to delay advising the shot for older adults, because of a lack of trial data in those age groups, may have contributed to hesitancy in taking the vaccine in parts of the continent.

And reports of a rare but sometimes fatal blood clotting condition following use of the vaccine in younger adults, mostly women, has prompted some countries to restrict its use to only older members of their populations. Some countries, such as Denmark, have scrapped use of the vaccine altogether.

AstraZeneca will release its first quarter financial results on Friday.

CNN’s Lindsay Isaac contributed to this report.