Freedom of Information (FOI) requests to 145 councils have revealed widespread differences in the proportion of looked-after children diagnosed with autism.
Among councils that responded, 19 (13 per cent) did not know whether any of their looked-after children had been diagnosed with autism, while six declined to respond.
Of the 120 councils that did supply figures, two said none of their looked-after children had been diagnosed. This includes the Corporation of London which had just eight children in its care when the request was made, and also Calderdale Council, which had a children in care population of 301.
A further two councils, Sandwell and Oldham, reported figures below the national prevalence rate of 1.1 per cent. When the request was made they had looked-after children populations of 587 and 476 respectively.
In contrast, Walsall Council reported that 81 (12.7 per cent) of its 636 looked after children population had been diagnosed with the disability, which affects how a person communicates and relates to other people and the world around them.
The average rate of autism diagnosis among councils was three per cent.
The FOI requests were made by the parent of an autistic child with experience of the care system, who is in talks with academics to use the findings as a basis for further research.
The parent, who asked to remain anonymous, said the findings give weight to the argument that councils are either failing to properly record the number of children in care who have autism, or that children in care are being under-diagnosed in the first place.
"Many (councils) expressed misgivings about the accuracy of their records," she said.
"Some local authorities' believe it is the responsibility of another agency to record information on numbers of children in care within the local authority with a diagnosis of autism."
She said that some councils may be "autism-blind" by failing to pursue a diagnosis as they see the disability's symptoms, which include behavioural problems, as being "emotional or trauma related".
As a result, many children are receiving inappropriate support to address perceived problems such as attachment issues, rather than necessary support for autism, she said.
"Autism awareness and specialist autism training must be a priority for workforce development within professions and within local authorities so all become better at understanding and meeting the needs of autistic children including autistic looked-after children," she said.
She has also called for councils to link up with local health professionals to collect accurate information related to their autistic looked-after children population, and for the Department for Education to work with the Department of Health to do more to encourage councils to collect such information.
National Autistic Society policy manager Tim Nicholls said: "This is deeply concerning and means that some children go through the care system without a full understanding of their needs or the right support in place, which could have a serious effect on their long-term prospects.
"Every local authority, who act as parents to children in care, has to assess each child's needs as early as possible and we believe this should include identifying autism.
"With more than 1 in 100 people on the autism spectrum, it is almost certain that every local authority will have an autistic child in their care.
"It's crucial that all local authorities should examine reported levels of autism among looked-after children - especially if they are low - and make sure they have the right systems in place to identify autistic children and their needs.
"Being autistic can be difficult but we've seen again and again how awareness, understanding and early support can make all the difference."