A report by Her Majesty's Inspectorate of Prisons (HMIP) on Coldingley Prison in Surrey, published this morning, shows that the practice of slopping out – where prisoners defecate into buckets – persists. Juliet Lyon, director of the estimable Prison Reform Trust, is right to call the practice "demeaning and unacceptable". The findings highlight the urgent need for the Justice Secretary, Ken Clarke, to resist pressure from the Tory right and the populist press, and to push ahead with his reforms of the prison system.

HMIP applauded the education provision, health facilities, and "solid focus on resettlement" at Coldingley. But it also expressed concern at sanitation arrangements "which led to the throwing of human waste out of windows". Inmates have restricted access to toilets during the night. This should be seen in the context of a report published in August by the National Council for Independent Monitoring Boards, which said there are around 2,000 cells in Britain, spread across 10 prisons, where slopping out is the norm.

This practice is shameful and incompatible with the professed ambition of our prison service to rehabilitate offenders in humane conditions. For several decades, Britain has locked up non-violent offenders, and recipients of short-term sentences, in defiance of advice from prison governors. This is unaffordable, ineffective, and frequently uncivilised. It has led to a prison population of more than 85,000 – the highest, as a proportion of the population, anywhere in the EU. Worse, 60 per cent of those jailed for less than 12 months reoffend within a year.

Mr Clarke uses economic expediency to justify his plans to reduce our prison population by 3,000. Labour's £2.4bn prison building and maintenance programme was halved in last week's Spending Review. He wants short sentences to be used "only when necessary", and inmates to work a 40-hour week on a minimum wage. Both ideas are commendable. Our penal system ought to shift its focus from punishment and deterrence to rehabilitation. For those in Whitehall and beyond who still need convincing, these reports of slopping out are a grim but useful place to start.