Privatisation of the probation service could become a "race to the bottom on price at the expense of effectiveness and efficiency", a union has warned.
The move is "bound to compromise public protection", with fewer staff, less supervision and less training, said the probation union Napo.
Launching its campaign against the proposals at its 100th annual conference, Napo said the shambles of the Olympics security contract with private firm G4S showed there was a clear need to overhaul the tendering process and the contracts themselves.
Harry Fletcher, the union's assistant general secretary, said: "This tendering exercise is not about quality, but purely about ideology and cost. In reality, it is a race to the bottom on price at the expense of effectiveness and efficiency, and is bound to compromise public protection."
Core probation services such as the supervision of criminals and the writing of pre-sentence reports could be put out to competition under Government plans.
Michael Spurr, chief executive of the National Offender Management Service (Noms), wrote to all 35 probation chiefs in England and Wales last summer, saying the Government was "examining the potential for the core probation services to be competed".
The move aims to improve and modernise public service delivery as part of David Cameron's Big Society. But Napo said the handing-over of the supervision of offenders to a private security company for the first time in July marked "the end of the probation service".
Serco, along with the London Probation Trust, won the contract to run community payback schemes across the capital in a move which will save £25 million.
The Government's decision to appoint Jeremy Wright as the prisons and rehabilitation minister, as opposed to the prisons and probation minister, was also a "clear signal that the role of probation is set to diminish", Napo added.
A Ministry of Justice spokesman said: "The public's safety is our top priority. We are extending competition to bring innovative, quality services to reduce reoffending, provided by those best placed to do so, whether they are in the public, voluntary or private sectors."