The number of autistic people in England could actually be double the official current figure, according to a new study.
Researchers found many adults are left undiagnosed, especially those over 50 with more than nine in ten of this age group without a formal diagnosis.
The team from University College London estimate there are around 1.2 million autistic people in England against a government figure of 700,000.
The groundbreaking research was published in The Lancet Regional Health Europe.
Now researchers are calling for better access to diagnostic services for adults, and better support after diagnosis.
Famous autistic people include Elon Musk, singer Susan Boyle, actors Sir Anthony Hopkins and Darryl Hannah as well as environmentalist Greta Thunberg.
Autistic people may exhibit differences in their social communication and social interaction, alongside restricted and repetitive patterns of behaviours, interests and activities.
Lead researcher, Dr Elizabeth O’Nions said: “Historically, autism has been considered as a condition of childhood.
"But recently, awareness has been growing that it is present across the lifespan – in adults as well as young people.
“Nevertheless, autism is still under-recognised in adults.
"Our estimates suggest that about 180,000 people aged 20-plus had an autism diagnosis as of 2018, meaning that most autistic adults in England were undiagnosed.
“This matters because autistic people often experience discrimination and exploitation in society.
"They may have unmet support needs, even when they appear to be coping with life.
“Having a diagnosis means that someone can advocate for their right to reasonable adjustments and the support they need.
"Recognising that someone with an intellectual disability is autistic can also help people to understand and support them better.”
The UCL team calculated the number of people who had received an autism diagnosis using anonymised data from more than five million individuals registered at GP practices in England between 2000 and 2018.
They then compared these figures with a lower (around one per cent) and upper (around three percent) estimate of how common autism really is in the population.
The upper estimate was based on rates of diagnosed autism in young people (aged 10-19) in the researchers’ dataset as this would be a more accurate distribution as more people are being diagnosed at that age.
The team’s estimates suggest that between 150,000 and 500,000 people aged 20 to 49 years-old may be autistic but undiagnosed.
Meanwhile, between 250,000 and 600,000 autistic people over the age of 50 may be undiagnosed more than 9 in 10 of all autistic people in that group.
The midpoint of these figures translates to approximately 750,000 undiagnosed autistic people aged 20 and above, in England.
This brings the total autistic population to over 1.2million – approaching double the figure of 700,000 cited by the government.
Dr O’Nions added: “Our findings indicate that there is still a substantial diagnostic gap in adults compared to children and young people when it comes to autism in England.
“This may partly reflect a lack of awareness and understanding of autism in adults on the part of healthcare professionals.
"Older adults may also be less likely to self-identify as autistic, meaning that they do not come to the attention of services.
“Meanwhile, providers may be hesitant to raise the issue of autism given the uncertainty around waiting times for a diagnosis and the availability of support or specialist services post-diagnosis.”
