The number of prison staff who have tested positive for coronavirus has surpassed the number of confirmed cases among inmates for the first time as the number of deaths continues to rise.

Covid-19 cases have been confirmed in more than half of jails in England and Wales.

A total of 341 prisoners have tested positive in 72 jails – and 18 have died as of 5pm on Wednesday, the Ministry of Justice (MoJ) said.

The latest figures for Covid-19 in relation to prison staff testing positive is worrying as it has steadily increased and that is a reason for more testing for staff as quickly as possible before the virus increases in our prisons

Some 364 prison staff have now contracted the virus in 64 jails and six have died.

There have also been 11 prisoner escort and custody services staff who have tested positive.

Steve Gillan, general secretary of the prison officer’s union the POA, told the PA news agency: “The latest figures for Covid-19 in relation to prison staff testing positive is worrying as it has steadily increased and that is a reason for more testing for staff as quickly as possible before the virus increases in our prisons.

“It also means that any discussion about relaxation of regimes must be treated with extreme caution to ensure the health and safety of both staff and those in our care.

“Prisons could potentially become an incubator for the virus in the coming weeks and months therefore we need to be realistic and keep regimes restricted until we see a fall in staff and prisoners testing positive.”

Over the past two weeks, 2,000 staff have been referred for testing, Justice Secretary Robert Buckland said.

At least 700 had been tested and more than 6,000 were self-isolating, according to the latest available figures.

Earlier this week, he acknowledged a shortage of some personal protective equipment (PPE) for prison staff but said there was a large delivery on order which was expected soon.

The news comes as prison charities abandoned legal action after claiming measures taken so far to address coronavirus behind bars by the Government were “unlawful” because they will have a “manifestly insufficient impact”.

The Howard League for Penal Reform and the Prisons Reform Trust (PRT) threatened a judicial review if urgent action was not taken to address their concerns after it emerged up to 15,000 prisoners may need to be released to keep the remaining staff and inmates safe.

Since then updated advice has been published which indicates the recommended figure has been reduced to nearer 5,000.

PRT director Peter Dawson said the campaigners will continue to “apply pressure”, adding: “We are not giving up.

“The need for there to be many fewer people in prison will not change and we will continue to make that case. The information we have gained this week because of the threat of legal action will be crucial in doing that.”

The trust said the Government’s letter in response – which it has called to be made public – is “very detailed and demonstrates that serious thought and consideration has been given to answering our concerns”, adding: “It is a real concern that two charities had to threaten legal action to receive this information. For prisoners and their families, openness and transparency is critical.”

On Thursday, Mr Buckland confirmed that “about 40” prisoners had been freed from jail early as part of efforts to control the spread of coronavirus behind bars.

Earlier this month, the Ministry of Justice (MoJ) pledged to release up to 4,000 low-risk prisoners on temporary licence to ease overcrowding, as well as pregnant inmates and mothers with babies.

All offenders must pass a risk assessment before their release is approved.

There were plans to release a “significant number” over the last week but the programme had to be put on hold after six inmates were released by mistake due to an “administrative error” before being returned to prison, the MoJ said.

In another plan, low-risk offenders could be moved into temporary prison cells in the grounds of seven jails so fewer are sharing.

In Kent, some prisoners are being relocated to a closed-down youth custody centre to allow more inmates to have a cell on their own.

As of Friday, the number of criminals behind bars in England and Wales was 81,124, having fallen by around 1,000 in a fortnight.