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.
Many people referred for assessment are being forced to wait too long to access autism-specific support services, as the latest figures from NHS England show a backlog has built up across the country.
As of the end of March, all of the 25 adults and children waiting for an autism assessment in NHS Bassetlaw CCG area had been on the list for more than 13 weeks – the longest time someone should wait for a diagnosis following a referral, according to national guidance.
This was an increase from the 15 patients waiting longer than 13 weeks at the beginning of March 2020, before the coronavirus pandemic.
Mr Nicholls, head of influencing and research at the National Autistic Society, a charity supporting those with the condition, said a diagnosis can be "life-changing" and is crucial to getting the right help and advice.
He said: “Without proper long-term funding for diagnosis services across the country, we fear that the waiting list will continue to grow and people could be left waiting months or even years for a diagnosis.
"For many of them, this will mean struggling without support at school, work or home."
Across England, there were more than 100,000 people waiting for an autism diagnosis as of the end of March, including 82,000 who had been waiting for at least 13 weeks.
This is a significant rise from before the pandemic – at the start of March 2020, 53,000 people were waiting to be assessed, 42,000 of them for more than 13 weeks.
A spokesperson for the NHS said it was seeing "record numbers" of people coming forward for support, and that the health service is working to meet increased demand on autism services.
Mr Nicholls cautioned while the latest figures are "important", they do not give a full picture of just how long people are having to wait after 13 weeks, for instance.
He said greater investment is needed in autism services, and that the Government must do more to reduce assessment wait times.
A Department of Health and Care spokesperson said: “We are investing £5m over the next two years to reduce diagnosis waiting times for autism for children and young people across the country.
“This is in addition to £13 million extra funding we invested last year, which will help develop a national framework with NHS England to ensure children, young people and adults receive higher-quality and faster diagnoses.”