An NHS target to reduce the number of people with learning disabilities being treated in long-stay hospitals has been missed, resulting in more than 2,200 remaining in-patients in England.
Charities say they are at risk of abuse and neglect in hospital units and should be cared for in homes in the community, close to their families.
NHS England committed to halving the numbers in hospitals, but achieved 19%.
It said it was investing in community care to help hundreds more people.
Figures for March show that there are at least 2,260 people with learning disabilities or autism staying in hospitals in England - most for around two years, with half being kept in secure wards.
The NHS target was to reduce numbers to 1,300-1,700 by now - from 2,805 in 2016.
The commitment was a response to the abuse scandal exposed by the BBC at Winterbourne View specialist hospital near Bristol in 2012, when six care workers were jailed for abusing patients.
NHS England said it was committed to supporting people with a learning disability or autism, and more than 650 people who had been in hospital for more than five years were now being supported in their communities.
It said parts of the country were hitting their targets, helping people to live more independently, closer to home.
But charity Mencap said "hundreds of people with a learning disability who should have been living in the community are still trapped in in-patient settings".
The charity said families were often powerless to get them out and people could be locked up for years on end, instead of being supported in their communities.
'Human rights scandal'
Dan Scorer, head of policy at Mencap, said: "This is a domestic human rights scandal.
"We want government to prioritise social care reform and demand that the NHS and local authorities work together to reunite families and develop the right support across social care, health and education for children and adults with a learning disability in their communities."
Children with learning disabilities are also sent to these hospitals, known as assessment and treatment units (ATUs) - and there were 240 patients under 18 being treated in these units last month.
This is an increase from 110 in March 2015, although there have been fluctuations in numbers.
'Kept like an animal in a cage'
Caroline Corfield's son Philip is 21 and has severe autism, learning difficulties and epilepsy.
From 2015 to 2018, he lived in several different care homes and ATUs - the most recent one in Hull, more than 200 miles from his home in Croydon. Philip stayed there for 22 months.
Caroline said the first time she visited him was the worst.
"He was being kept like an animal in a cage - he was in a seclusion room where he was being fed through a hatch."
She was shocked by the fact his carers told her they were only spending two to three minutes interacting with him at a time.
"I felt like he was slipping away because the longer you leave him the harder it is to reintegrate him into society."
In 2018, he started getting support in the community and is now living in his own home close to his family.
Caroline says her son is much happier and more affectionate now.
"I can just walk into the house and see him and Philip can just give me a big hug.
This is a contrast to his time spent in a secure unit, she says.
"I didn't feel like my opinions were being taken into account and I didn't feel like they were putting anything in place to make things better for him."