The European Medicines Agency (Ema) said its safety committee “has started a review of a safety signal to assess reports of thromboembolic events” with people who had received the shot.

“Four serious cases of unusual blood clots with low blood platelets have been reported post-vaccination with Covid-19 Vaccine Janssen,” the Ema said, referring to US pharma giant J&J’s European subsidiary.

“One case occurred in a clinical trial and three cases occurred during the vaccine rollout in the US. One of them was fatal.”

The watchdog has approved the Johnson & Johnson vaccine for use but its rollout across the 27-nation EU is not due to start until later this month.

The probe comes days after the Amsterdam-based watchdog said it was listing the same type of blood clots as a very rare side effect of the rival AstraZeneca vaccine.

Both jabs use similar adenovirus vector technology.