Government plans to lock up fewer criminals will fail to cut costs or re-offending, according to a former Home Office criminologist.

Professor Ken Pease said "community sentences as currently delivered have no evident effect on rates of reconviction".

Using them to replace short prison sentences simply "freed the group most likely to reoffend to do so sooner, with no evidence of a current treatment benefit from community sanctions to offset that," he said.

In his report, Prison, Community Sentencing and Crime, released by the think-tank Civitas, Prof Pease said it was important for any move away from the use of custody "to be based on something more than short-term political exigency".

But Metropolitan Police Commissioner Sir Paul Stephenson said he was "rather fond of villains going to prison" - including for crimes such as burglary which can carry sentences of less than 12 months.

The Government appears committed though with Justice Secretary Kenneth Clarke questioning the link between rising imprisonment and falling crime earlier this year, saying it was "virtually impossible" to rehabilitate offenders on short sentences.

"Banging up more and more people for longer without actively seeking to change them is what you would expect of Victorian England," he said.