Julian Assange says he and other inmates at HMP Belmarsh have been placed into lockdown in their cells after being told there has been a Covid-19 outbreak in his block.
The WikiLeaks founder said a number of inmates at the Thamesmead prison have received a letter from the governor notifying them that there had been a number of positive Covid-19 cases.
All prisoners and staff are to be swabbed to detect for Covid-19 at the prison in the next 24-48 hours, said Assange.
He added that all exercise has been stopped, showers have been prohibited and meals are to be provided direct to prisoners in their cells.
Assange has been held at Belmarsh Prison for the last year and a half and is awaiting the judgment from a US extradition hearing on 4 January.
A Prison Service spokesman said: "We've introduced further safety measures following a number of positive cases." It is understood that one wing of the prison is affected.