There was no evidence, it said, that services to prisons and conditions of imprisonment were "significantly improved in privately operated facilities".

In fact, staffing in private prisons was 15 per cent lower than in public prisons, management information systems were less well organised and the number of major incidents higher.

Private prisons also had a higher rate of assaults both on prisoners by other prisoners and prisoners against prison staff.

In Canada, the Government returned the country's one private prison to the public sector in 2006, saying that after five years "there had been no appreciable benefit". An assessment of overall performance found that the publicly run correction centre "performed better in key areas such as security, health care and reducing reoffending rates".

The UK experience is even more instructive. A 2003 report by the National Audit Office in the UK found that the performance of private prisons had been "mixed". The best were better than most public prisons, but the worst were at the bottom, among the least well performing public prisons.

Most British prisons were privately owned till the late 1870s, when they were nationalised amid concerns that commercial gain and the just treatment of prisoners were incompatible. Penal reformers argued that running a prison wasn't quite like running a mill - but that thinking had lost ground by 1992, when the prison system was again part-privatised.

As Tony Blair argued in 1993: "I have to say that I am fundamentally opposed both in principle to the privatisation of the prison service and indeed in practice ...

"I believe people who are sentenced by the state to imprisonment should be deprived of their liberty, kept under lock and key by those who are accountable primarily and solely to the state.

"There is a danger that if you build up an industrial vested interest into the penal system, and as part of that interest they are designed obviously to keep the prison population such that it satisfies those commercial interests ... there is a risk that that distorts the penal policy that otherwise you would introduce."

A leading campaigner against prison privatisation, Stephen Nathan, adds that "privatising prisons requires more people in the criminal justice system for longer and is squarely at odds with the public good".

The point is not how strong a business case we can mount for private prisons; this is about the morality of running prisons for profit, and the proper division between public and private sectors.

British prison expert, Professor Andrew Coyle: "The real issue is not about whether private prisons are managed more effectively than public prisons, or vice versa. The fundamental change which has come about with the introduction of privatisation is the concept of prison as a 'marketplace' and a business which will inevitably expand."

By Tapu Misa | Email Tapu