Proposals to reduce the number of children being diagnosed with autism are being considered by NHS commissioners in south-west London.
The idea of restricting an autism diagnosis to only the most severe cases was discussed earlier this month.
The local alliance of five clinical commissioning groups (CCGs) says nothing has been decided yet.
But the National Autistic Society says it is "deeply concerned" about the plans.
Board papers from a meeting of south-west London and St George's mental health trust a fortnight ago describe a service under intense pressure, with waiting times for autism diagnoses of more than 10 months.
The team is supposed to carry out 750 assessments a year.
But it is getting almost double that level of demand, with about 25 referrals a week.
The papers state that NHS commissioners, who fund services, have asked the trust to review who is eligible for the service - so they focus specifically on children who have another condition such as depression.
The report goes on to say: "The effect of reviewing and revising the criteria must be to reduce the number of children and young people who are able to access a full diagnostic assessment from the trust."
'Strain on families'
The National Autistic Society has written in protest to the local CCGs, covering Kingston, Merton, Richmond, Wandsworth and Sutton.
The charity's head of policy, Sarah Lambert, said: "If it goes ahead, this will leave many local children without access to a diagnosis and unable to access the specialist support they desperately need.
"That will threaten their long-term prospects and put a greater strain on many already vulnerable families.
"We believe this goes against CCGs' legal duty to make sure children in their area can access a diagnosis service.
"An autism diagnosis can be life changing. It can explain years of feeling different and help unlock crucial advice and support."
The charity has written to commissioners to express its concerns and to offer to help them make sure that this service remains available.
It ran a national campaign two years ago, highlighting the difficulties and long waits autistic people and families face in many areas in getting a diagnosis.
'No change this year'
SEND Family Voices, the parent/carer forum in Kingston and Richmond, said: "The sense is that these proposals have been agreed, without due consultation with any of the stakeholders - and more importantly, without consideration of the impact upon families, children and young people."
The five CCGs said: "No decision has been made by the Trust or by the commissioning CCGs on taking this approach forward.
"We absolutely recognise the concerns of families of children with a neuro-developmental condition and the groups who support them.
"Any potential changes to the service would require detailed engagement with stakeholders. The CCGs are fully committed to talking to local people before reaching any conclusions on the way forward.
"We want to be clear that nothing has been decided by the CCGs or changed at this time and no change will happen in the next 12 months."