Research conducted by charity Ambitious about Autism found that from their child first being referred for an autism assessment, 46 per cent of parents waited longer than 18 months for a formal diagnosis.
Some parents reported waiting for more than three years.
Children are usually referred by a GP or health visitor for a formal assessment if they suspect the child is autistic. They should then have a multi-disciplinary diagnostic assessment by a team of professionals that can include a paediatrician, a speech and language therapist, and a specialist psychologist.
A total of 3,842 parents and carers of autistic children responded to the survey, which was conducted between September and November 2019.
Parents also reported struggling with a lack of support after an autism diagnosis – with more than 70 per cent saying the support they received was not good enough.
In many cases parents reported being left “completely alone” without a follow-up appointment or signposts to sources of support, such as speech and language therapy.
The survey findings come just two months after MPs warned that lengthy waits for assessment and diagnosis for children with learning disabilities can result in them not getting the support they need and potentially being detained in "brutal" and "horrific" conditions in mental health facilities.
Jolanta Lasota, chief executive of Ambitious about Autism said: “Many parents say they feel isolated and helpless as they wait to find out if their child has autism.
“To make matters worse, our research tells us that even when families receive an autism diagnosis, they continue to struggle to access adequate help to understand and support their child’s needs and help them feel positive about the future."
Ambitious about Autism has launched an online toolkit designed to help parents navigate autism in the early years.
“Autistic children have so much potential but we know that the earlier they can benefit from the right support and interventions, the easier it is for them to thrive and achieve as they grow up," Lasota said.
One parent, Helen from Kent, spent two years waiting for diagnosis for her son Evan, now four.
“I didn’t think I could go to the GP with concerns about my son's development," she said.
"I lived this life day in day out, worrying he might have something like brain cancer, or wondering if I was just a bad mum, and not knowing how to help him.”
Helen said that receiving a diagnosis gave her the confidence to seek support in the community, and further her understanding of her son.
“Having a resource that shows parents what to look out for and how they can raise their concerns is really important," she said.
"It’s also important parents know where to look for support and what their rights are.”
A Department of Health and Social Care spokesperson said: “Autistic people deserve a timely diagnosis, in line with Nice guidelines, to help them access the right support.
“Increasing the help available is one of the top priorities in our NHS long-term plan, which commits to reducing waiting times for children and young people.
“We will also publish a refreshed autism strategy later this year to ensure it is fit for purpose and are extending the scope of the strategy to include children.”