The supervision of high-risk criminals who have committed notorious offences that have attracted significant and prolonged media attention will not be handed to the private sector when 70% of the probation service is outsourced this year.

Criminal justice experts are concerned the system has been devised so that all the risk is passed to the remaining rump of the public probation service.

Leaked Ministry of Justice "restricted policy" draft guidance on the allocation of 220,000 offenders in England and Wales between new private and voluntary sector providers and a residual probation service shows the public sector will be left to deal with all high-risk offenders, such as murderers and serial sex attackers, and those cases where there is an exceptional public interest in retention by the public sector.

The rules also show that if the case involves a celebrity or an offender whose identity will attract significant media attention, management approval is needed before their supervision can be handed to a private rehabilitation firm, even if the case is assessed as low to medium risk and the offender has not been given a jail term.

The disclosure of the complex ground rules for the allocation between public and private sectors of all the criminals in England and Wales currently supervised by the probation service comes as a fresh attempt is made in the Commons on Tuesday to require a vote of both houses before the privatisation of the probation service can go ahead.

The leaked MoJ draft guidance details an initial bureaucratic procedure to determine whether each offender is high, medium or low risk which includes a new highly complex "risk of serious recidivism" calculator. It also outlines a secondary procedure for the 25% of cases where the risk rating is expected to escalate from low or medium to high risk during the course of their punishment.

Probation unions say the new procedures will involve a massive increase in paperwork and may compromise public protection as probation staff spend even less time actually supervising offenders.

The National Association of Probation Officers (Napo) said that currently staff allocate cases on the basis of risk of harm under a process that takes a few minutes.

The union claims the new system to decide whether the case will be handed to one of the 21 new community rehabilitation companies will take at least 45 minutes. The leaked document says the process, which has to be undertaken for all 220,000 offenders, should be completed "within one working day" for each case.

Napo claims the escalation procedure, which could involve 55,000 offenders a year, will take three to three and a half hours – seven times longer than at present.

Napo's general secretary, Ian Lawrence, said: "The supervision of offenders is a highly demanding and complex task. It is essential that as much time as possible is spent in face-to-face contact with offenders constantly assessing and reassessing their risk rather than sat in front of computers. This is a highly dangerous strategy."

Criminal justice expert Harry Fletcher said: "The case allocation system has been devised so that all risk is carried by the public sector. Even cases which are deemed low risk but which might attract media attention will be held by the public sector."

Justice minister Jeremy Wright said: "Around 85,000 crimes are committed every year by prisoners released from short sentences.

"We must act now to address the scandalous gap that allows our most chaotic offenders to leave prison with no support or supervision to turn their lives around. For anyone to complain about more extensive risk assessments and greater supervision of these chaotic offenders would be a pretty strange position to take."

The justice secretary, Chris Grayling, told MPs last month that a pretty rapid process of assigning existing probation staff between public sector and the future community rehabilitation companies was already under way.

"The aim is to have those two teams in place by April and to have started the process of migrating cases so they are properly allocated across those two groups, but without an absolute requirement to do so by 1 April, because we need to take it carefully and do it over a period of time," Grayling told the justice select committee.

"We need to make sure public safety is guaranteed. If a particular offender is in the wrong group but there is a good reason for them staying with the current offender manager, they will do so."

Grayling said his aim was to thoroughly road test the new system with a period of dry running in the public sector and to have it in place by December 2014.