Labour has accused ministers of blocking publication of an internal risk assessment of probation changes which will see about 70% of rehabilitation work handed to the private sector and voluntary organisations.
The Ministry of Justice has rejected a request under the Freedom of Information Act for the risk register to be published, the Opposition said.
Shadow justice secretary Sadiq Khan said: "Because of the Government's secrecy, we don't know whether the public's safety is being endangered. What have they got to hide?"
The Labour frontbencher accused the Government of taking "an enormous gamble with public safety" by going ahead with Justice Secretary Chris Grayling's policy.
He said: "There is no evidence that their ill-thought through plans will work. Instead of rushing ahead they should be looking for evidence of what works to reduce re-offending. What makes it worse is their refusal to publish their own assessment of the riskiness of their half-baked plans. Labour has requested sight of their risk assessment but ministers have blocked publication."
Mr Khan added: "At a time when the Government is considering allowing big private companies to run our courts and to take over some roles from the police, they also want the supervision of dangerous offenders taken over by global corporations more interested in profit than public safety."
Labour peers will attempt to amend the Offender Rehabilitation Bill to ensure the plans are debated in the Lords. The amendments call for the plans to proceed only after they have been fully trialled and independently evaluated to make sure there are no risks to public safety.
The changes, dubbed by the Government as a "rehabilitation revolution", will see a greater role for private and voluntary-sector organisations, which will be paid by results to reduce re-offending.
The Government says the reforms are necessary to tackle re-offending as more than 58% of prisoners serving fewer than 12 months commit further crime within a year of release.
The Government is to undertake the biggest re-organisation of the prison estate in more than 20 years, creating a nationwide network of around 70 ''resettlement prisons'' so nearly all offenders are released into the area in which they will live and be supervised. It will make it harder for offenders to move home while they are under supervision to ensure continuity in the support they receive.
www.justice.gov.uk/(Ministry of Justice)