Some high-risk offenders in England and Wales may not have been monitored as closely as they should have been during the lockdown, a report suggests.
An internal Ministry of Justice document shows probation staff did not carry out planned checks in about half of cases, during one four-week period.
The National Probation Service (NPS) has insisted supervision was adequate.
It comes as the government is expected to announce the NPS will take over the probation system in England and Wales.
This would end the involvement of private companies.
Since the coronavirus lockdown, the state-run NPS has scaled back face-to-face supervision of thousands of sex offenders and violent criminals, to prevent infection.
Emergency plans were drawn up for most offenders to be contacted by telephone or visited on their doorstep.
But, according to the MoJ document seen by BBC News, in the four weeks to May 17, only 51% of high-risk offenders under supervision had the contact that had been planned for.
The statistics also show that during the week leading up to May 17, 18% of high-risk prisoners did not have immediate appointments with probation officers on release.
They should have had a meeting within one “business day” of leaving jail.
However, the NPS disputed the significance of the figures.
A spokesperson said: “This data is partial, experimental and unreliable.
“We don’t use it in isolation to judge performance and the public shouldn’t do either.
“All our wider evidence in combination shows offenders are receiving the right levels and types of supervision.”
Private firms, known as community rehabilitation companies – which supervise low and medium-risk offenders in England – had planned contact in 61% of cases during the four weeks, according to the data.
Under government plans, outlined in May 2019, the companies would have lost their offender supervision role to the NPS – but would have been able to bid to run unpaid work schemes, drug misuse programmes and training courses.
But ministers have halted the process and are believed to have decided that the NPS should deliver rehabilitation services as well as managing the entire caseload of offenders.
However, it is thought there will still be opportunities for voluntary groups and charities to operate specialist offending behaviour schemes.