Almost half of parents whose children have been referred for an autism assessment have to wait 18 months or more for a formal diagnosis, according to a survey.

The charity Ambitious about Autism polled almost 4,000 parents of children with autism and found that in the most extreme cases families reported waiting three years for a diagnosis.

Even after a diagnosis is made, many families go on to struggle to get the right help for their child’s needs, the charity says. “Many parents say they feel isolated and helpless as they wait to find out if their child has autism,” said Jolanta Lasota, the chief executive.

“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.”

According to Ambitious about Autism, about one in 100 children in the UK have autism, with four times as many boys diagnosed as girls. Of those parents who took part in the poll, 49% said they waited 18 months or longer after referral to the point of diagnosis. A further one in five (19%) said it took between 12 and 17 months.

About 70% complained they were not offered adequate support after diagnosis, while a number reported being left without a follow-up appointment or additional information about support groups.

“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 Kent mother described waiting two years for a diagnosis for her four-year-old son. Receiving the diagnosis gave her the confidence to seek support.

“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.”

The charity has launched an online toolkit for parents of children aged five and under that offers information on assessments, diagnosis, development and support available.

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.”