Inmates at a Lancashire prison have been transferred after a serious outbreak of Covid-19 and the death of a member of staff.
Jo Farrar, head of Prison and Probation Service, told a hearing of the Commons Justice Committee some prisoners had been moved from HMP Wymott in Leyland.
The Ministry of Justice (MOJ) would not reveal the number of cases there.
It has confirmed 207 prisoners in 57 jails have tested positive with 13 suspected Covid-19 related deaths.
Half of prisons in England and Wales have confirmed cases of coronavirus.
Ms Farrar told the online hearing it was "very hard to guarantee" that inmates who had the virus but did not have symptoms would be held apart from others.
However, she said some Wymott prisoners had been transferred because of a serious outbreak.
A MOJ spokesperson said a small number of "vulnerable" inmates had been moved from HMP Wymott and it was looking at ways of further increasing single cell occupancy there.
"We have robust and flexible plans in place to keep prisoners, staff and the wider public safe based on the latest advice from Public Health England."
A 48-year-old staff member from HMP Wymott died on 8 April although the MOJ said it could not confirm his death was caused by coronavirus.
HMP Wymott is a male Category C prison which holds more than 1,100 prisoners with approximately half convicted of sex offences.
The MOJ is planning to release up to 4,000 prisoners in England and Wales early in an effort to control the spread of coronavirus.
Two staff members at Pentonville Prison in north London died after showing symptoms of coronavirus.