Justice Secretary Chris Grayling insisted yesterday that it was safe to sell off the probation service, despite a leaked government assessment saying the opposite.

Mr Grayling told the justice select committee that the report was "theoretical" and refused demands to release other internal risk assessments into the sell-off.

Under the government's plans up to 70 per cent of the probation service would be sold off.

Private and voluntary groups have been asked to bid for contracts worth £450 million to supervise 225,000 low and medium-risk offenders a year under the shake-up - including murky security corporations Serco and G4S.

The leaked "risk register" drawn up within the Ministry of Justice showed a more than 80 per cent risk of "an unacceptable drop in operational performance" leading to "delivery failures and reputational damage."

Labour MP John McDonnell said it meant that under the changes, which have already led to a strike by probation staff, the general public "could be put at risk."

Mr Grayling said the risks had been revised since the register was drawn up, although the timetable for privatisation remains the same and no explanations have been offered on how they have been mitigated.

"It's just good management practice. That's all the risk register is," he said.

He also denied that the sell-off posed safety issues, claiming that "at no point has any official come to me and said 'if you go ahead with this there is an 80 per cent chance of the public being put in danger.'"

Contracts are to be split across private Community Rehabilitation Companies (CRCs) in 20 English regions and one Welsh region, while the new public sector National Probation Service (NPS) will be formed to deal with the rehabilitation of 31,000 high-risk offenders each year.

Probation officers union Napo is fighting against the plans, arguing that they will trash valuable interagency relationships needed to deal with offenders properly.

Napo spokeswoman Tania Basset said Mr Grayling's reticence to publish the risk registers "indicates that the reviewed documents are still flagging dangers up."