People in Slough, Bournemouth and Poole are not being sufficiently protected from young offenders, a report said.
The Inspectorate of Probation rated four Youth Offending Teams (YOT) as part of a three-year program to inspect all 157 areas in England and Wales.
It rated provision in Slough "worse than ever" and Bournemouth and Poole's services as "very disappointing".
The Ministry of Justice (MoJ) said it will set out plans shortly to improve youth rehabilitation services.
In the two other areas reviewed, work in Medway, Kent, was deemed "average" with public protection work scoring 61% and efforts to reduce reoffending scoring 71%.
Meanwhile Milton Keynes scored "a creditable set of findings" with 69% in its work to protect the public and 71% in its work to reduce the likelihood of reoffending.
A re-inspection report on the Bournemouth and Poole team stated work to minimise the risk of a young offender harming others was "done well enough 36% of the time", and "work to make each individual less likely to re-offend was done well enough 45% of the time".
Andrew Bridges, HM Chief Inspector of Probation, said: "We consider this a very disappointing set of findings."
He called the Slough Youth Offending Team's inspection "a worse than average set of findings", with its public protection work scoring 54% and work to minimise reoffending done "well enough 61% of the time".
An MoJ spokesman said: "We are determined to ensure all YOT are performing well and protecting the local people they serve.
"The Youth Justice Board will work with the local authority of poor-performing YOTs on a programme of action to make improvements.
"The MoJ has consulted on proposals to improve youth rehabilitation and prevent reoffending. We will set out our plans shortly."