What a difference a few days make. Some 48 hours after it recorded a massive 27 per cent leap in pre-tax profits, outsourcing giant Serco was told by a High Court judge that one of its children’s prisons was ‘woeful’ and ‘wholly inadequate’.

Ashfield Prison, near Bristol, won’t be holding children for much longer – the government announced in January that it should be re-rolled as an adult prison – and Mrs Justice Nicola Davies’s judgment, handed down last week, reveals why this change cannot come a minute too soon.

The judge ruled that Ashfield unlawfully punished seven boys after they were involved in a protest over conditions on their wing in February 2012. They were kept in isolation, and five of them were subject to a regime known as ‘restriction on the wing’ – a shadow segregation practice which was unlawful because it lacked all the safeguards which apply to formal segregation procedures.

The boys were denied a fair trial when Ashfield imposed additional days on their sentences. That was a breach of the European Convention on Human Rights, found the judge, who condemned the prison for its ‘wholly inadequate system’ for disclosing case papers. Senior staff had a ‘woeful absence of knowledge’ about their legal duties, she added.

Ashfield broke the law again by banning the boys from the prison gym. This is more serious than it sounds. Going to the gym is one of the few opportunities boys in prison have to exercise. Take that opportunity away and you get trouble. An independent inquiry, led by Lord Carlile, into the use of force on children in custody found that a lack of exercise leads to disorder.

The boys were punished after they were involved in a protest on an astro-turf at the prison. The incident was witnessed by a district judge, who was visiting the site in his role as an Independent Adjudicator.

The judge was invited to recuse himself from either conducting the boys’ adjudication hearings or passing sentences, but he declined. He imposed additional days’ imprisonment on each of the boys – 90 days in total. These additional days were only taken off after the Howard League for Penal Reform, which represented the seven claimants, began legal proceedings.

Details of the case are alarming, but they should come as little surprise to anyone who has studied Ashfield’s record.

Ashfield is the most violent prison in the country, with more than 1,000 assaults recorded there last year. In fact, one in 15 assaults in the entire prison system, including adults, happens at this jail hidden away in leafy Pucklechurch, in south Gloucestershire.

Inspectors reporting in February 2012 said that physical restraint had been used at the prison almost 150 times a month – a nine-fold increase compared to their previous visit.

In one month alone, there were 480 strip-searches of children – an intrusive, degrading and traumatic practices that many other prisons have stopped. The inspectors found no evidence that they led to the finding of anything that wasn’t allowed in the jail.

Figures released by parliament show that boys at Ashfield were kept in solitary confinement 377 times during 2011 – double the number recorded in 2008.

Last year the Howard League totted up the additional days each youth jail in England and Wales had imposed on the children they held. Ashfield’s shameful statistic – a total of more than five years’ imprisonment between January 2010 and April 2012 – was far higher than all the others.

None of the seven claimants has won a payout. Indeed it is highly unlikely that any of them will receive compensation. The Howard League does not take civil compensation cases, and we have not referred or advised clients to take up such a claim.

So this judgment isn’t about money. For the seven boys at the centre of the case, it is an acknowledgement that their treatment and the regime they were subject to was unfair and unlawful. It shows that their concerns were entirely justified.

Our legal action has already had a positive effect. During the course of the court proceedings in December, Ashfield amended some of its local policies, particularly in relation to adjudication and training of staff. A month later, the government announced that the prison would be re-rolled.

The judgment should serve as a warning to other children’s prisons that they cannot make the mistakes that Ashfield did.