Autism is a lifelong condition which impacts how people communicate and interact with the world.
It is normally diagnosed at a young age, although some may receive a diagnosis as teenagers or into adulthood.
The National Autistic Society expressed concerns over the long waits, which it said can increase patients’ “likelihood of reaching crisis point” and called on the Government to invest in diagnosis services.
NHS Digital figures show around 445 adults and children suspected of having autism in Bath and North East Somerset, Swindon and Wiltshire were waiting at least 13 weeks for a diagnosis in June. Of those, 35 were under 18.
This meant 72.4% of 615 patients registered with the NHS Bath and North East Somerset, Swindon and Wiltshire Integrated Care Board in the month had experienced delays beyond the 13-week recommended limit.
This was an increase on last year, when those who waited beyond the recommended time during the same period accounted for 68.1% of all 470 patients.
Across England, more than 143,000 people were waiting for an autism assessment in June, around 20,000 more than last year. Of those, more than 118,000 had a referral that had been open for longer than the recommended time.
Mel Merritt, head of policy and campaigns at the National Autistic Society, said waiting lists will continue to grow unless urgent long-term funding for diagnosis services is provided.
She added: "People often can’t get the right help and support without an autism diagnosis, and long waits for diagnosis and support can leave people in a difficult situation and increase their likelihood of reaching crisis point.
"The Government must invest in diagnosis services, as set out in the national autism strategy, to reduce waiting times and ensure all autistic children, young people and adults get the support they need."
In Bath and North East Somerset, Swindon and Wiltshire, around 40 patients seen in June had waited more than 13 weeks to get their first appointment.
A Department of Health and Social Care Spokesperson said: “We know it’s vital to have a timely diagnosis of autism. NHS England recently published a national framework and operational guidance to set out how children, young people and adults can receive a timely assessment, and we’ve made £4.2 million available this year to improve services for autistic children and young people.
“We expect Integrated Care Boards and NHS Trusts to follow the clinical guidelines on autism published by the National Institute for Health and Care Excellence (NICE) and this year, we are also updating the Statutory Guidance on Autism to support the NHS and local authorities to deliver improved outcomes for autistic people.”