Mentally ill prisoners in the largest high-security jail in the UK face “totally unacceptable” delays in being transferred to a secure hospital, leading to a further deterioration in their conditions, inspectors have said.

Inmates with severe illnesses in HMP Wakefield in West Yorkshire, which has held some of the country’s most high-profile sex offenders and murderers, were not receiving the care they needed, Her Majesty’s Inspectorate of Prisons (HMIP) said.

Peter Clarke, the chief inspector of prisons, has taken the unusual step of writing directly to the prisons minister, Rory Stewart, to urge him to take action to ensure transfers under the Mental Health Act are completed within the target of 14 days.

Clarke said the problem of delays in transferring prisoners to secure hospitals was not unique to Wakefield but “was particularly acute there”.

“Because of the totally unacceptable delays in doing so, many prisoners across the prison estate are held in conditions that are not in any way therapeutic and indeed in many cases clearly exacerbate their condition,” he said. “This is a national strategic issue to which we have made reference many times.

“The situation at Wakefield was yet another example of prisoners with severe illness not receiving the care that they needed. It is clearly something that is beyond the capability of either individual prisons or HM Prison and Probation Service (HMPPS) to resolve.

“Therefore, in view of the fact that to date there has been no effective response to this issue, on this occasion I am taking the unusual step of making a recommendation directly to the prisons minister in the hope that he can use his influence to initiate effective cross-departmental action to address the problem.”

Inspectors observed one prisoner who was “exceptionally challenging to manage and had complex needs that could not be met in the prison”.

“While staff attempted to manage him positively and constructively, his condition was deteriorating during a lengthy wait to be admitted to a secure hospital,” the inspection report said.

Wakefield prison dates back to 1594 when it was a “house of corrections”, but most of the current buildings date from the Victorian period.

Nearly all of its 700prisoners have been convicted of sexual or violent offences and 60% were serving a life sentence or an indeterminate sentence for public protection. Almost half were aged 50 or over and the oldest man was 91.

found thatthe prison was overall “calm, secure and decent”. . Inspectors found an “impressive set of initiatives and good work” which could be shared across the prison service as good practice.

The report said it was a respectful prison, with many examples of good relationships and interactions between staff and inmates. “By any standards this was a good inspection, which was highly creditable given the complexity of the prison,” Clarke said.

Michael Spurr, the outgoing chief executive of HMPPS, said: “I am pleased that the chief inspector has commended the high quality of work being done at Wakefield which is a credit to managers and staff. The governor will use the recommendations in this report to further develop the establishment to meet the needs of its prisoners.”