Sixty-nine prisoners were released from British jails by mistake last year.
Sex offenders, burglars and inmates convicted of violence and robbery in England, Wales and Scotland were among those wrongly freed.
The statistics for 2009-10 were disclosed to the BBC after a Freedom of Information Act request.
The majority of criminals mistakenly freed were sent back to jail, but five are understood to be still at large.
It is thought most mistakes were due to prison staff miscalculating release dates.
The figures also revealed that since 2007, 39 prisoners serving sentences for violence, assault or robbery were freed by mistake, as well as six sex offenders.
A spokesman for the Prison Service of England and Wales said most prisoners released in error were returned to custody quickly.
“The number of releases in error remains very small as a percentage of total discharges. This type of error is a rare but regrettable occurrence,” he said.