LONDON: Britain's medicine regulator has approved Johnson & Johnson's COVID-19 vaccine for use, the health ministry said in a statement on Friday, making it the fourth COVID-19 shot available for use in the country.

Britain also cut its order for the single-dose shot, also known as J&J unit Janssen's vaccine, amid issues with the company's supply chain and reports of rare blood clots.

Britain has given two-thirds of the population a first dose of COVID-19, and the government cited the "unprecedented scale and pace" of the roll-out as behind the decision to cut its order to 20 million doses from the original order for 30 million doses.

"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.

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.

Britain has advised that under-40s are offered an alternative to the AstraZeneca shot, and the government said that vaccine advisers the Joint Committee on Vaccination and Immunisation (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.