Nearly half of people in prison in England and Wales could be warehoused in 1,000-plus supersized jails under government plans to transform the prison estate, the latest edition of the Prison Reform Trust's Bromley Briefings Prison Factfile reveals.

The pressure of budget cuts and economies of scale have led to the roll out of "Titan prisons by stealth" with a drive to close small community prisons, build larger jails and add additional capacity to existing establishments.

This is despite evidence published by the Prison Reform Trust and included in the briefing, based on data provided by HM Prisons Inspectorate, showing that smaller prisons tend to be safer and more effective than larger establishments, holding people closer to home and with a higher ratio of prison staff to prisoners.

The government plans to build a 2,000-place prison in Wrexham and is conducting a feasibility study for a second giant-sized institution in West London. Since 2010 there have been 13 closures of smaller prisons and a further six still to come.

In January the Justice Minister Jeremy Wright announced plans to open up an additional 1,260 places in four new house blocks across the prison estate. The first one at HMP The Mount is on track to accept prisoners in September 2014. In addition he announced plans to open 180 new places at Rochester and Bure this year.

On current trends the proposed changes will result in around 38,000 people held in 30 supersized jails across the country, the Prison Reform Trust's analysis of the latest prison population statistics and projections reveals. This represents nearly half of the total number of people behind bars in England in Wales.

Of the 84,000 people currently in prison, 33,300 are held in establishments of 1,000 or more - 40% of the total prison population. The number of supersized jails has nearly trebled in the past decade with 28 out of 124 prisons in England and Wales currently holding over 1,000 men. Ten years ago only 11 prisons had numbers of over 1,000 holding 18% of the total prison population.

The top three supersized jails are all in the private sector: G4S-run Oakwood (1,600) Birmingham (1,436) and Sodexo-led Forest Bank (1,348). A full list of 1,000 plus prisons is published below.

A separate Prison Reform Trust briefing, published in 2008, drawing on a comparison of large and small prisons based on 154 factors by the Prisons Inspectorate, revealed that larger institutions are consistently poorer at meeting prisoner needs and creating a healthy prison environment.

It found that a prison with a population of 400 prisoners or under was four times more likely to perform ‘well’ than a prison with a population of over 800. People in large prisons found it easier to get hold of illegal drugs and were more likely to say that they had been assaulted or insulted by a member of staff or by another prisoner. They were less likely to receive help with housing or know who to contact to get help with a drug addiction or finding a job.

Lord Woolf, Chair of the Prison Reform Trust, in his seminal report on the prison system following the disturbances at Strangeways prison, recommended prisons “should not normally hold more than 400 prisoners … the evidence suggests that if these figures are exceeded, there can be a marked fall off in all aspects of the performance of a prison.”

The trend to larger prisons appears to mark a reversal of policy for the Prime Minister David Cameron. Speaking in 2009 on the then-Labour government's plans to build five new 2,500-place Titan prisons, which were subsequently cancelled, he said: "the idea that big is beautiful with prisons is wrong."

The recent disturbing HM Prisons Inspectorate report into Oakwood prison, housing 1,600 people at one third of the average cost per prisoner place, ought to give the government pause for thought in the rush to build ever larger establishments with ever lower staffing levels, the briefing suggests.

The briefing warns that the pace and scale of change in the justice system and the impact of budget cuts are placing prisons under unprecedented strain and could undermine government plans to transform rehabilitation.

Last week, the Chief Inspector of Prisons Annual Report 2012-13 highlighted a worrying decline in the quality and quantity of rehabilitation available to prisoners. “All the establishments we inspected were under pressure to do more with less and, in some, the cracks were beginning to show,” the Chief Inspector Nick Hardwick said.

The rapid round of prison building, closures and re-roles, the re-designation of up to 70 establishments across England and Wales as resettlement prisons, a risky review of the incentives scheme and a punishing benchmarking process, plus outsourcing most probation and resettlement services to the private and voluntary sector, are all happening at a time when the National Offender Management Service has to make overall resource savings of almost 25% in real terms by 2014-15. The spending review saw a further 10% reduction in the Ministry of Justice’s budget.

Meanwhile, on the government’s own estimates, a looming Offender Rehabilitation Bill and an Anti-Social Behaviour, Crime and Policing Bill threaten to add hundreds, if not thousands, to the prison population.

Commenting, Juliet Lyon, Director of the Prison Reform Trust, said:

“Minds could be concentrated by severely limited resources. Instead the pressure of budget cuts and economies of scale have led ministers to go for ever larger and cheaper-to-run jails at the expense of prisoners’ safety and rehabilitation.

“Rehabilitation is a worthwhile goal but complex problems require complex solutions. The needs of vulnerable people should concern every department of national and local government, not be monopolised by Justice. Prisons cannot, and should not, continue to pick up the tab for a range of social and health needs.

“A more effective and far-sighted use of taxpayers’ money would see addicts receiving treatment in the community, or in residential centres, and people who are mentally ill, or those with learning disabilities, getting the health and social care they need to lead responsible lives in their communities.”

A shorter summary version, Prison: The Facts, is available for iPad and iPhone on the App Store and for Android devices via Google Play.