Visits to Magilligan prison near Limavady have been temporarily suspended in response to a coronavirus situation.
It is understood a number of health workers who attend the jail have tested positive for Covid-19.
So far, two prison staff and two inmates are self-isolating as a precaution while they await test results.
Magilligan is a medium-security jail housing around 450 male inmates.
'Established processes in place'
For a period earlier this year, visits were halted at all Northern Ireland prisons as a precautionary measure during lockdown.
In a statement, the Prison Service said: "We can confirm a number of people from a partner organisation have tested positive for Covid-19.
"The service has well-established track and trace processes in place and these have been instigated.
"As a consequence, in-person visits at Magilligan have also been temporarily suspended.
"The safety of everyone at Magilligan remains our primary focus."