It should not be possible to make profits out of prisons. The power to lock people up, depriving them of their liberty and separating them from their families, is a responsibility that should be the preserve of the state. Yet a pro-market ideology has seen private companies become responsible for about one in seven of the UK’s 92,000 prisoners – a proportion second only to Australia. Allowing companies to make money out of punishing people, which is what prisons are for, along with rehabilitation and public protection – was a bad idea when it started under John Major’s government in the 1990s, and remains one today.

This is a point of principle, one based upon the idea, evidenced by international studies, that private investment would distort public policy against more lenient sentencing and discourage moves to prevent reoffending. An analysis of official UK prison data by the Guardian suggests there are also practical reasons to object to existing private prisons, and oppose the government’s plans to build at least two more. Our report revealed that the level of violence in private jails is far higher than in public ones. This seriously undermines a key defence of private jails. Comparisons across the whole estate must be handled with caution. Some private prisons, including HMP Oakwood in Staffordshire, have been praised by inspectors. But the finding that assaults in private, male-only local jails are far more prevalent than in their public equivalents must not be brushed off by ministers. While the 28 public local men’s jails – housing prisoners directly after sentencing or on remand – recorded 493 assaults per 1,000 prisoners in the year to September 2018, the five private jails of the same type recorded 701 – which is 42% higher.

To believe that nationalising prisons would solve all the extremely serious problems that now confront the sector would be a misjudgment, however. The spectacular failure of HMP Birmingham and its re-absorption into the state from contractor G4S put private prisons in the dock. Yet failures are mirrored at state prisons including Liverpool and Nottingham. In some instances, private-sector management practices and incentives may have contributed to making a bad situation worse. The appalling state of prisons is well documented: rising levels of violence, self-harm, squalor and drug-taking, and increasing numbers of prisoners spending too many hours locked in their cells. The main reason for the appalling state of many prisons is the toxic combination of overcrowding and cuts.

Between 2010 and 2015 prisons lost a quarter of their budget, and nearly 30% of their staff. The result is that many British prisons are literally falling to pieces, with long backlogs of repairs. While the overall decline in staffing levels has been reversed since a low point in 2016, thanks to £104m in increased funding, one in three officers were reported last year to have been in the job for less than three years. How private and public prisons compare with regard to staffing is obscure because staffing levels in private prisons are secret, a lack of transparency that is another reason to object to outsourcing.

Meanwhile the number of prisoners is expected to rise further. Violence is at record levels, with 33,803 assaults in the year to September 2018. Former prisons minister Rory Stewart promised to resign this summer if he had not made a “measurable difference” in 10 key prisons. Instead he was promoted. The justice secretary, David Gauke, is in the process of unravelling the disastrous part-privatisation of the probation service by his incompetent predecessor Chris Grayling. This should improve the overall situation, since effective supervision and community punishments are an essential component of a working criminal justice system. But sentencing policy and practice too will need to change before there can be any end in sight to the unfolding disaster in Britain’s jails.