'Bedroom tax will affect 80,000' London families
Up to 80,000 "low-income families" in London will be affected by the government's "bedroom tax" plans, the National Housing Federation says.
The group said some would lose an average of £771 a year in benefits if they have a spare bedroom in their council or housing association home.
It said it was "shocking" the change would affect 50,000 disabled people.
The government says it needs to end the "spare room subsidy" to help those in overcrowded homes.
The Discretionary Housing Payments fund, which critics have called a "bedroom tax", would see people having their benefits reduced if they are deemed to have a spare bedroom in their council or housing association home.
Those with one "spare" room would lose 14% while those with two or more would lose 25%.
The changes are due to come into place on 1 April.
The NHF said people who share the care of their children, families where young children have a small bedroom each, foster carers and disabled people who have their home specially adapted for their needs, would be affected by the change.
Michelle Smith, London lead manager for NHF, said: "The government's bedroom tax is flawed and will unfairly penalise thousands of people across London who have lived in their homes for years, raised families and contributed to their communities."
She said in most areas of London, there were not enough smaller affordable homes for families to move into.
Ms Smith added that many would have to move into more expensive privately rented properties, which would add to the overall housing benefit bill, and said ministers should build more affordable homes instead to cut the benefits bill.
The Department for Work and Pensions said there were 380,300 households on social housing waiting lists in London, and that there were 88,000 overcrowded homes in the private sector and 126,000 overcrowded homes in the social sector.
"Councils have been given an extra £155m this year so that they can help their vulnerable tenants with £30m specifically targeted towards supporting disabled people and foster carers," a spokesman said.
"However, with over a quarter of a million tenants living in overcrowded homes and two million on housing waiting lists, we need to end the spare room subsidy and ensure a better use of social housing."