About half of parents must wait 18 months or more for their child to be formally diagnosed with autism, a charity warned.
Ambitious about autism, many mothers and fathers feel "isolated and helpless" while awaiting diagnosis.
Many families then struggle to get the right help for their children's needs when a diagnosis is finally made.
A charity survey of nearly 4,000 parents of children with autism found that 49% waited 18 months or more from the reference point for a formal diagnosis.
Another in five (19%) said it took between 12 and 17 months.
And about seven in 10 (70%) believe they did not receive adequate support after diagnosis, such as additional information about autism, support groups, or therapies, such as speech and language help.
Jolanta Lasota, executive director 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 are diagnosed with autism, they still struggle to get adequate help to understand and support their children's needs and help them feel positive about the future. "
Lasota added that previous autistic children receive the right support and interventions, the easier it is for them to "thrive and reach as they grow up."
Ambitious About Autism has launched a new online toolkit to provide parents with information, tips and checklists to help them support their children.
The charity claims that about one in 100 children in the UK has autism, with four times more boys than girls diagnosed.