* Britain reduces order of J&J shot by 10 million doses
* J&J Janssen shot the fourth COVID-19 vaccine to be approved
* J&J has faced issues with supply, rare blood clots (Adds reaction, detail of booster trial)
By Alistair Smout
LONDON, May 28 (Reuters) - Britain's medicine regulator on Friday approved Johnson & Johnson's Janssen COVID-19 vaccine for use, with the government adding it 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 as the country's vaccine rollout progresses, while the company has also warned of uncertainty over its supplies to Europe.
"This is very welcome news and another boost to our hugely successful vaccination programme," 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.
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 programme later this year," health minister Matt Hancock said.
The vaccine is part of a British study looking at whether giving third-dose booster COVID-19 shots could extend immune protection.
"The results of this will be important in determining how supplies of this vaccine can be best utilised when they arrive," said Professor Arne Akbar, President of the British Society for Immunology, adding the approval would not have an immediate impact on rollout.
J&J's vaccine had already been approved by the World Health Organization, the United States and the European Union, where reports of rare blood clots are being reviewed.
The shot uses similar viral vector technology to the Oxford-AstraZeneca vaccine, which has seen reports of similar clots.
British officials have advised that under-40s are offered an alternative to the AstraZeneca shot due to the link to clots, and guidance on how J&J's shot is used will be issued before rollout begins later this year. (Reporting by Alistair Smout; editing by Michael Holden, Kate Holton and David Evans)