A fifth of convicted criminals in Caerphilly County Borough reoffend within a year, official statistics show.
Prison reform advocates warn that a revolving door of short sentences for repeat offenders has led to cramped jails and a multibillion-pound bill for taxpayers.
Ministry of Justice figures reveal that 21% of the 1,420 adults released from prison, cautioned or handed a non-custodial conviction at court between October 2016 and September 2017 in Caerphilly committed at least one further crime within 12 months.
Between them, the 304 reoffenders racked up 1,004 new offences – an average of three each.
They had each committed 16 previous crimes on average, according to the data.
The reoffending rate was even higher among children.
Of the 114 offenders aged under 18 in Caerphilly, 36 (32%) carried out another crime in the year following a court conviction, caution, reprimand or warning.
Across England and Wales, reoffending costs the public an estimated £18 billion each year.
Nationally, 29% of adult offenders in the October 2016 to September 2017 cohort reoffended within 12 months, rising to 39% for juveniles.
Reoffending rates also vary considerably depending on both the type of offence and length of sentence.
They have remained high, at around 62%, for adults released from prison sentences less than 12 months.
Citing this high rate of reoffending, former justice secretary David Gauke called for short jail terms to be scrapped earlier this year.
The chief probation inspector, Dame Glenys Stacey, has also criticised the “expensive merry-go-round”, but stressed that scrapping short sentences would not reduce reoffending on its own.
Frances Crook, chief executive of the Howard League for Penal Reform, said: “Cramming more and more people into prisons is a recipe for squalor, violence, drug abuse and mental distress – and, as these figures show, ultimately more crime.
“Introducing an assumption against short prison sentences, as has been implemented in Scotland, would better protect the public because evidence published by the Ministry of Justice shows that short bursts of imprisonment lead to more offending and more victims.
“For children, the evidence is even clearer. The more contact a child has with the criminal justice system, the more entrenched they are likely to become – and this pushes up offending rates.”
The MoJ said while the youth reoffending rate has increased slightly over the last decade, the number of children entering the justice system has dropped dramatically.
A spokeswoman added: “Reoffending creates more victims of crime and costs society over £18 billion a year – that’s why we’re creating a system that can rehabilitate offenders while ensuring robust monitoring takes place in the community.
“In order to achieve this we are giving offenders the skills and support they need to succeed in the outside world, while our probation reforms will make sure licence conditions are enforced consistently.”