By Valentina Pop

BRUSSELS -- The European Union, seeking to catch up with other parts of the world for access to coronavirus treatments, agreed with AstraZeneca PLC to buy as many as 400 million doses of the U.K. drugmaker's Covid-19 vaccine now in development.

The deal, for which no value was given, is the EU's first such pact with a drugmaker, although some members of the 27-country bloc have already secured future vaccine supplies. The U.S., China and the U.K., which this year left the EU, in recent months have struck deals AstraZeneca.

AstraZeneca's AZD1222 vaccine is now in clinical trials, with the first batches expected to be available next month in the U.K. The vaccine is being developed with University of Oxford researchers.

Under the deal announced on Friday, the EU will make a down payment for the vaccine doses on behalf of the entire bloc. When the vaccine becomes available, EU countries will need to divide the doses among themselves and pay for the actual product, said Darragh Cassidy, a spokesman for the European Commission, the EU's executive arm.

The commission is also close to a deal for up to 400 million vaccine doses from Johnson & Johnson and for 300 million doses from the French-British alliance of drugmakers Sanofi SA and GlaxoSmithKline PLC, Mr. Cassidy said. Talks with other drugmakers developing coronavirus vaccines are also under way, he said.

Vaccine negotiations mark a new role for the commission, which has little involvement in EU health care because it remains in the hands of national governments. The coronavirus pandemic and the resulting economic crisis have prompted EU members to give the Brussels-based commission more authority to coordinate and oversee the bloc's response.

Soon after the pandemic hit Europe, France, Germany, Italy and the Netherlands launched independent negotiations with AstraZeneca, but handed those talks off to the commission to pursue on behalf of the entire bloc and its roughly 450 million residents, Mr. Cassidy said.

The EU down payment will come from a EUR2.7 billion ($3.2 billion) emergency funding the bloc has set aside for coronavirus vaccines. Some of the EU's doses could also be donated to needing countries outside the EU.

"This strategy will enable us to provide future vaccines to Europeans, as well as our partners elsewhere in the world," said European Commission President Ursula von der Leyen.

The commission said it would use all available legal tools to speed EU certification of successful vaccines, including by easing rules on labeling and packaging.

Write to Valentina Pop at valentina.pop@wsj.com