It is "astonishing" that the Government is considering more privatisation schemes in the probation service, the union Napo has said.
Initiatives involving the private sector in probation work in the past have been problematic or have failed, it said.
"The Government clearly wishes to privatise unpaid work and hostels and look at the feasibility of putting out to tender probation programmes and even an entire probation trust," it said.
"The Probation Service is also facing unprecedented cuts as a consequence of the Government's U-turn on sentencing reform. The Government says it believes that the introduction of market forces would increase efficiency and effectiveness. In reality the strategy is about saving money, is ideologically driven, will reduce standards, introduce profits and reduce community accountability," it added.
Assistant general secretary Harry Fletcher said: "The Probation Service is to be savagely cut and primed for privatisation in the near future, however the criminal justice system does not lend itself easily to the laws of supply and demand or market forces.
"Ministers therefore have been consistently forced to create pseudo-markets in order to force through privatisation."
He added: "It is astonishing that the Government is considering even more privatisation schemes, ignoring the experience over the last decade of market forces not working in a criminal justice setting. Offenders are individuals; no matter how serious their offence they are not commodities."
A Ministry of Justice spokesman said: "We want community sentences which are tougher for offenders and better value for the taxpayer. The Probation Service plays an important role in rehabilitating offenders but we must look at the best options to cut re-offending. We are developing innovative new tools to achieve this, like payment by results.
"The organisations chosen to run Community Payback will have to consult and can involve local groups, including charities and voluntary organisations, when deciding what work offenders undertake.
"No final decisions have been made on competing out approved premises and there is no evidence to suggest that savings made to the Probation Service will compromise public protection."