Cuts to housing benefit for families with disabled children are "ludicrous", "unfair" and "appalling", according to a group of medical experts speaking exclusively to Channel 4 News.
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The British Academy of Childhood Disability (BACD), which represents the UK's foremost child disability experts, says the so-called bedroom tax is "astounding".
From April, 660,000 social housing tenants face a reduction in their housing benefit because they are deemed to be under-occupying their homes.
According to the government's own figures, 420,000 of those affected have some form of disability.
Under the rules, couples and children of the same sex are expected to share a room.
There is no exemption for families caring for children with severe disabilities.
Consultant Paediatrician John Gibbs, from the BACD, said the new measure would "penalise disabled children."
"To impose this tax, this so-called tax on families because they have a disabled child is astounding and it's appalling," he said.
"It sounds ludicrous because a lot of these disabled children have had bedrooms specially adapted or in many cases specially built for their needs.
"If the parents can't afford to pay for this new so-called bedroom tax, they have to move to a smaller house, they lose their specially adapted bedroom and then they have to sleep with their brother or sister."
"The parent may be reluctant to attend to that child regularly if it's going to wake the other sibling."
Mr Gibbs said he was also concerned about the impact on the brothers or sisters of disabled children if they were forced to share a bedroom.
"The effect on the other brother or sister would be a totally disruptive night, sleep deprivation and could have a psychological impact as well," he said.
"Children with significant disabilities have many medical problems and particularly at night, many have mobility problems, many have epilepsy and seizures can be particularly troublesome at night."
"For all these medical reasons disabled children often have discomfort during the night and they need much care and attention from their parents and that would be highly disruptive for any brother or sister that had to share the bedroom with them."
Read more: 'Bedroom tax' - the key questions
Rachael Davies, from Cleator Moor in Cumbria, is astonished that her daughters Saskia, 13, and Skye, 14, are expected to share a bedroom.
Saskia has a rare brain condition which requires round the clock care. She is unable to speak or use the toilet and has severe epilepsy, attention deficit disorder and autistic tendencies.
Rachael says downsizing to a two-bedroom home would be a "nightmare".
"There would be no independence for Skye. Saskia herself would be wrecking Skye's stuff," she said.
Skye says the idea of sharing a bedroom with Saskia is "silly".
"She's noisy next [door] to me anyway. You can hear her downstairs, so being in the same room you wouldn't get any sleep at all. We've been in a hotel. She kept jumping on me and laughing to herself. It would hit me after the first day. I couldn't imagine sharing a bedroom with Saskia."
Look at 'individual cases'
David Cameron was again pressed on the issue at Prime Minister's Questions.
Mr Cameron accused Labour of "rank opportunism and irresponsibility" in attacking the proposals.
However, he promised he and the Department for Work and Pensions would "look at any individual case".
Mr Cameron also pointed to a Discretionary Housing Fund, which will be administered by local authorities, "to support people affected by the under-occupancy measure."
Channel 4 News surveyed every council in the North of England to ask what level of support they would be able to provide.
Most of those who replied indicated discretionary payments could only be offered to a small percentage of tenants affected, and that support would generally be short to medium term.
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