More than 100 staff at Cardiff Prison either have coronavirus or are self-isolating as contacts of someone who does.
It is understood that 113 staff are affected at the category B male prison.
Sources have told WalesOnline that the huge number of employees were unable to work because they had either tested positive or were self-isolating.
Around 800 men are held at the prison on Knox Road.
The Ministry of Justice said the prison is operating safely and extra support has been brought in.
A Prison Service spokesman said: “Our priority is to limit the spread of the virus and protect the lives of those who live and work in our prisons.
“We have taken precautionary measures at the prison, in line with public health guidance, and will continue to closely monitor the situation.”
WalesOnline asked The Ministry of Justice (MoJ) how many prisoners at HMP Cardiff have coronavirus, but the MoJ refused to comment.
However, last week a number of cases at Cardiff Crown Court had to be adjourned because of defendants - who were on remand or awaiting sentencing at HMP Cardiff - had tested positive for the virus.
Although specific figures for HMP Cardiff are not available, the number of prisoners testing positive for the virus has increased in prisons in Wales and England - according to the MoJ's own statistics.
At the end of October, 1,529 prisoners had tested positive for Covid-19 since March, an increase of 883 compared with September.
Earlier this month, UK Prisons Minister Lucy Frazer said there had been "significant challenges" but insisted lessons had been learned as the year progressed and "we have taken every opportunity to refine our strategy -and will continue to do so".
