A prison for young adults has been placed in special measures by the government after it was found that inmates were locked alone in their cells for up to 23 hours a day – a practice campaigners have described as de facto solitary confinement.

Aylesbury Young Offender Institution in Buckinghamshire also kept some inmates in its segregation unit for up to three months at a stretch. The practice raised “significant concerns for their mental and physical wellbeing”, said independent monitors.

Last week the Ministry of Justice placed the prison, which holds 440 inmates, mostly aged between 18 and 21, in special measures, in a bid to improve safety and living conditions.

The move came after the Prisons and Probation Ombudsman upheld complaints on behalf of three inmates who had been segregated or locked in their cells. As a result, an internal review of Aylesbury’s segregation unit has been carried out and the prison has formally apologised to at least one of the three prisoners.

The Howard League for Penal Reform, which made the complaints to the ombudsman, expressed alarm at the “systemic practice” of keeping young adults in conditions amounting to solitary confinement.

It received almost 200 calls to its confidential line for young people in custody from prisoners at Aylesbury over a 12-month period to 25 January. Almost a third related to segregation.

Laura Janes, the league’s legal director, said it had been “inundated with calls from distressed young adults isolated in their cells [at Aylesbury] for over 23 hours a day, often for weeks and sometimes months at a time”.

They described “feeling bored, frustrated and sometimes even suicidal”, she said. “They often have no idea when their isolation will be brought to an end, adding to their sense of hopelessness. We have had to make numerous complaints and safeguarding referrals to the prison, which appears to be in a perpetual state of crisis.

“It is well known that locking energetic young men in their cells for excessive periods of time can cause irreversible harm. It is unfair on them and the staff charged with their care, and does nothing to help prevent reoffending.”

The league said that telephone contact between prisoners and their families was being restricted as a result of the practice, prisoners were being forced to eat meals alone in their cells, and some were unable to shower for days. Last April, prisoners caused damage to a wing in a protest over lack of showers and time out of cells.

Facebook Twitter Pinterest David Lidington, the local MP, said it was clear that there was still more that could be done at Aylesbury YOI. Photograph: Brian Lawless/PA

The most recent official inspection of the prison, two years ago, found that between 30% and 42% of inmates were locked in cells during the day. “Many young men received less than an hour a day out of cell, contributing to frustration and poor behaviour,” said the report from Her Majesty’s Inspectorate of Prisons. “Debilitating staff shortages” were a contributory factor in prisoners being locked up for long periods, along with high levels of violence. Some prisoners did not want to leave their cells because they felt unsafe, it added.

An Independent Monitoring Board report, published in January, said violence was rife in Aylesbury YOI. The prison contained “some of the most disruptive and challenging young men in the prison system”, it said, with one in 12 serving life sentences.

Across the country, 38% of young adults spent less than two hours a day outside their cells, said the chief inspector of prisons in his 2018 report. Prisoners should be unlocked for at least 10 hours a day, he added.

The ombudsman’s investigation into “X”, one of the Aylesbury prisoners whose case was taken up by the Howard League, found that he spent most of each day in his cell “without meaningful contact” after being segregated for his own safety following an assault on another inmate. “It appears that, at most, [X] had a daily half an hour out of his cell for association … and 20 minutes’ shower time,” said the report, which has been seen by the Observer. “This is unacceptable.” A similar conclusion was found in the case of prisoner “Y”.

In his report on prisoner “Z”, who was held in the segregation unit for 12 weeks after assaulting a member of staff, the ombudsman said: “Segregation is the hardest form of imprisonment. It is accepted that for most prisoners, segregation has a negative impact on their emotional and mental wellbeing.”

Last year, the British Medical Association, the Royal College of Psychiatrists and the Royal College of Paediatrics and Child Health said there was an “unequivocal body of evidence on the profound impact solitary confinement can have on health and wellbeing”. There was “an increased risk of suicide or self-harm amongst those placed in solitary confinement”.

Terms used to describe the practice included isolation, segregation, separation, or removal from association. “Regardless of the term used, we consider any individual who is physically isolated and deprived of meaningful contact with others for a prolonged period of time to be in solitary confinement,” the organisations said.

The Howard League raised its concerns about Aylesbury YOI in a letter to the local MP, David Lidington. “It seems the prison is resorting to increasing use of punishment in a desperate effort to maintain control,” the letter said. Inmates were suffering from “misery at being held for weeks on end in almost total isolation and inactivity”.

Lidington said: “Aylesbury YOI holds many of the most difficult young prisoners in the country, most of whom have committed serious offences, and the staff work with great professionalism in a very challenging environment. They have told me about the efforts that they are making to enhance safety and make possible more opportunities for education and work that offer prisoners a better chance of holding down a legitimate job after release, although it is clear that there is still more that can be done.”

The Ministry of Justice said: “While we have placed Aylesbury in special measures, that simply means we have determined it needs additional support from the centre to drive improvements.” It said the prison had “changed its approach to segregation in response to the [ombudsman’s] recommendations. It has increased in-cell activity like education and now ensures that those in segregation can be involved in the physical education programme.”