Medicine regulator in the United Kingdom has approved Johnson & Johnson's Janssen COVID-19 vaccine for use, the health ministry reported on Friday, adding the government had cut its order for the vaccine by 10 million doses.

The shot is the fourth COVID-19 vaccine approved in Britain, and the government said it would be available for use later in 2021 without specifying when.

Britain reduced its order to 20 million doses from 30 million doses as the country's vaccine rollout progresses, while the company also faces issues with its supplies to Europe and reports of rare blood clots.

"This is very welcome news and another boost to our hugely successful vaccination program," Prime Minister Boris Johnson said on Twitter.

Britain has given two-thirds of its adult population a first COVID-19 shot, and the government cited the "unprecedented scale and pace" of the rollout as behind the decision to cut its order.

However, Health Minister Matt Hancock maintained that the J&J shot would still play a key role. Britain has also approved vaccines made by Pfizer, AstraZeneca and Moderna .

"As Janssen is a single-dose vaccine, it will play an important role in the months to come as we redouble our efforts to encourage everyone to get their jabs and potentially begin a booster program later this year," Hancock said.

J&J's vaccine is already approved in the United States and European Union, where reports of rare blood clots are being reviewed.

The shot uses similar viral vector technology to the Oxford-AstraZeneca vaccine, where similar clots have been reported.

British officials have advised that under-40s are offered an alternative to the AstraZeneca shot, and the government said that the Joint Committee on Vaccination and Immunization (JCVI) would also advise on how J&J's shot was used.

The government said the shot would be available later in the year. There have been reports that J&J expects to miss its delivery targets for the European Union this quarter.

(With input from agencies)