People with suspected autism are being forced to wait more than two years to be assessed and almost four years to receive their diagnosis, new NHS performance figures have revealed.

The long delays have been criticised as “scandalous” by the former health minister Norman Lamb, who obtained the figures, and “completely unacceptable” by the National Autistic Society.

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Under NHS guidelines, children or adults in England who are thought to be autistic are meant to be assessed within three months.

Statistics received by Lamb from NHS mental health trusts in England under freedom of information laws revealed a “postcode lottery”. In 2016-17 an adult had to wait 848 days, and a child 799 days, for their assessment at the Berkshire healthcare NHS foundation trust. Under-18s waited an average of 574 days for their first appointment at the trust.

No waiting time has been specified for those with suspected autism to be formally diagnosed. However, the Liberal Democrat MP’s research found that one adult waited 208 weeks, and a child 196 weeks, for a diagnosis by specialists at Somerset partnership NHS foundation trust.

“These scandalous waiting times are happening despite clear evidence that early diagnosis and intervention can make a massive difference to people’s life chances. In many cases a diagnosis is necessary for children to access vital support, including education, health and care plans”, said Lamb.

Long delays in diagnosis could badly affect patients and their families, said Mark Lever, the chief executive of the National Autistic Society.

“Many autistic people and parents describe their diagnosis as life-changing. It can explain why someone has always felt different, help unlock barriers to vital support and enable people to take control of their lives”, said Lever.

“But long waits are traumatic and pushing people into anxiety and depression. Autistic people and families are desperate for help and are breaking down under the strain.”

The main organisation representing NHS trusts blamed the delays on their lack of money and understaffing.

“NHS trusts and frontline staff want to provide the prompt, high quality care the public rightly expects”, said Miriam Deakin, the head of strategy at NHS Providers.

“It is vital that we address the funding and staffing shortages that lie behind these delays which affect children and adults, while giving long-overdue priority to children’s services to make the guidelines meaningful.”

The NHS has begun collecting data on waiting times for autism, with plans to publish them for the first time next April.

Lamb urged ministers to tackle long delays by introducing a new maximum length of time anyone suspected of having autism should have to wait before being diagnosed.

Experts said that diagnosis could take time because often patients will need to undergo several assessments before a specialist could form a view.

Children living in the area covered by the Northern, Eastern and Western Devon NHS clinical commissioning group also face long delays to be assessed. In 2016-17 a patient under the age of 18 had to wait 271 days to be seen by an NHS service, while in the Cumbria partnership NHS foundation trust area there were waits of 253 days.

The Department of Health maintained the NHS should ensure that everyone with suspected autism should be assessed within three months, regardless of where they live.

“People with autism should receive a timely diagnosis. That’s why, for the first time, we will collect and publish diagnosis waiting times and are working closely with NHS England to support local areas to improve the quality and efficiency of care”, a spokesperson said.

“The guidelines on autism are clear: families must start assessment within three months and we expect every part of the NHS to adhere to this.”