The number of rapes, murders and other serious crimes committed by offenders on parole has risen by more than 50% since reforms to probation were introduced four years ago, according to official data that has triggered calls for the government to rethink its plans for another shake-up of the service.

Serious further offence reviews – which take place when a convicted offender under supervision is charged with another serious offence (SFO) – rose from 409 in the year before the 2014 reforms to 627 in the 12 months up to last April.

The new figures for England and Wales – which were shared with Plaid Cymru’s justice spokeswoman, Liz Saville Roberts – come as it emerges that coroners have taken the highly unusual decision to reopen inquests into three people killed by offenders under supervision, a move that is expected to expose systemic flaws in the probation service.

The problems are blamed on former justice secretary Chris Grayling’s reform programme, which saw some probation work outsourced to eight private providers, who were given responsibility for running 21 community rehabilitation companies working with low and medium risk offenders.

“Since the private contracts were let there have been staff cuts of up to 30%, offices have been merged and the quality of supervision has fallen sharply,” said Harry Fletcher of the Victims’ Rights Campaign.

Ian Lawrence, general secretary of the probation union, Napo, said there was a “clear correlation” between the increase and the reforms, which he blamed on increased workloads, low morale and chronic staff shortages which have left the National Probation Service with more than 1,000 vacancies. “There will be further serious offences whatever system you run but we think the fragmentation of the service has been a serious factor in the increase in SFOs,” he said.

A ministry of justice spokeswoman said the reforms “had extended probation supervision to around 40,000 extra offenders each year” and therefore “analysis of the number of offences does not provide a like-for-like comparison”.

The Observer has learned that any lapses in the supervision of serious offenders are set to be examined in court following a decision to reopen inquests into the deaths of three people who were killed by offenders under supervision.

As a result, previously confidential documents, including serious case reviews and risk assessments, will be shared with the victims’ families.

Alex Malcolm, five, died in 2016, after being attacked by Marvyn Iheanacho who was in a relationship with his mother. Iheanacho had been convicted of attacks on previous partners and children. A condition of his supervision was that he was not to be left alone with a child.

Lisa Skidmore was raped and murdered in 2016 by Leroy Campbell, a registered sex offender who was under supervision on probation. A review revealed that six weeks before he killed Skidmore, Campbell had told his probation officer he was having feelings that were “troubling him” and mentioned rape.

Conner Marshall, 18, was beaten to death by David Braddon in south Wales in 2015. Braddon, who had taken a cocktail of drugs and alcohol, had missed eight separate probation appointments in the weeks leading up to the attack.

The coroners have agreed to resume the inquests so that lessons can be learned to prevent future deaths and because, it is argued, the state may have failed in its duty to safeguard the right to life.

Amid signs the system is struggling, the government is ending the contracts for the eight private providers two years early and reducing the number of community rehabilitation companies from 21 to 10. It is also spending £22m improving support for ex-offenders.

Saville Roberts accused the government of “wilfully wrecking the humanitarian principle of rehabilitation” and called for more probation services to be returned to government control. A coalition comprising the probation trade unions, the Probation Institute, the Howard League, the Centre for Crime and Justice Studies, and the Centre for Justice Innovation have written to the justice secretary, David Gauke, urging him not to rush into retendering the contracts until a thorough review of the probation service has been conducted.