Autistic children are being put at risk by lengthy waiting times for diagnosis and treatment, a report has warned.

Healthwatch Croydon said the lack of “effective support services” for young people with autism in the borough could lead to a “decline of physical and mental health”.

The patient watchdog published the report, Autism: The Experience of Children and Young People in Croydon, last week.

Some Croydon parents waited 18 months for a referral by their child’s doctor, much longer than in neighbouring boroughs.

A lack of post-diagnosis support led to children self-harming, binge-eating and experimenting with drugs, noted the report.

Gaining access to a social worker could also be difficult, said Healthwatch, which cited a family which called 10 times in one day without making contact.

The report added: “These delays in access costs the NHS more as for some, conditions become more serious, and circumstances deteriorate sometimes leading to hospital treatment and unnecessary stress for child and carer.”

There were 881 children with diagnosed autism in Croydon in 2012, a figure expected to rise to 1,414 by 2021.

Charlie Ladyman, chief executive of Healthwatch Croydon, said: “Waits of 18 months have a detrimental impact on the child’s development, with consequences for family, carers, and indeed society as a whole.

“We welcome the Croydon Clinical Commissioning Group’s planned redesign of the service particularly with 20 per cent increased funding for children and adolescent mental health services and a commitment to reduce waiting lists.

“However, this needs to be supported by extra resources for GP training and social support as well as better information and processes to build trust.”

A spokeswoman for Croydon CCG said “improving access to child and adolescent mental health services, including autistic spectrum disorder services”, was “a priority”.

She added: “We are working closely with all our partners, South London and the Maudsley NHS Foundation Trust, Croydon Council and the voluntary sector to improve mental health services for children and young people in Croydon.”