Jail sentence calculations do not have the “same level of competency” after UK government cuts, the Prison Governors Association president has claimed.
Between 2014 and 2018, 13 prisoners in Wales were released by mistake.
Andrea Albutt said due to cuts the role of calculating sentences has been shared rather than having a dedicated clerk, which can lead to more errors.
The Ministry of Justice said instances were rare and all prisoners who had been let out early had been recaptured.
More than half of those released by mistake were from HMP Cardiff, including one person being held on remand for violence who was not returned for more than a year.
Another prisoner serving five years for possession of weapons was released 81 days early from HMP Parc, Bridgend, according to figures obtained by the BBC in a Freedom of Information (FOI) request.
It was four months before they were found and returned.
There were no recorded instances of such mistakes at HMP Berwyn in Wrexham, Wales’ newest prison.
Ms Albutt, a former governor of Swansea prison, said: “One reason why releases in error occur is because we used to have a dedicated sentence calculation clerk, who would do them every single day and they would be very, very competent.
“Because of government austerity we don’t have the same level of competency because it just gets shared around a few different people.”
Ms Albutt said calculating sentences with care was “really, really complex”.
She added: “It’s not straightforward that someone is going to serve four years in custody. You need to have competent people doing sentencing calculations.”
But she said the numbers were not high enough to warrant public concern and “sentencing calculations in general are good”.
“It’s very unlikely for someone to be released who is serving a life sentence or someone five years early,” she said.
“Whenever a person is released from prison you would hope they don’t reoffend, that would be the same if they are released early.
“I don’t think it’s really a risk to the public.”
The MoJ said: “We work closely with the police to recapture offenders at large and investigate each of these incidents thoroughly to see what lessons can be learnt.”