The amount of housing benefits being paid to private landlords has doubled over the last 10 years, with the cost to taxpayers greatest in London and the South East, and housing associations blaming a shortfall in social housing.
According to figures from the National Housing Federation, almost one in three housing benefit recipients now live in a privately rented home, up from 25 per cent in 2008.
And the total benefits paid to landlords has doubled since 2006, climbing to £9.3bn.
If all tenants receiving housing benefit were in social homes, it would save the taxpayer upwards of £2bn a year, the NHF said.
And the cost is particularly high in London and the South East, where the gap between private and social rents is greater than anywhere else in the country.
The average weekly housing benefit award in the social rented sector is £89, the NHF said, or £110 for private rents.
But in London, average support for private rents is £188, up from £124 for social rents.
NHF chief executive David Orr said: “It is madness to spend £9bn of taxpayers’ money lining the pockets of private landlords, rather than investing in affordable homes.
“Housing associations want to build the homes the nation needs. By loosening restrictions on existing funding, the government can free up housing associations to build more affordable housing at better value to the taxpayer and directly address the housing crisis.”
However, the National Landlords' Association hit back, noting that the proportion of landlords letting to tenants on benefits has actually fallen as state support has not kept up with rents.
NLA chief executive Richard Lambert said: “The private rented sector plays a significant role in providing much-needed homes for tenants so there seems no real benefit in the NHF taking a cheap shot at landlords.
“What we should all be talking about is the failure of successive governments to adequately allocate its housing budget and to incentivise the building of new homes. In the long term, that would be the best use of taxpayers' money”.
A spokesman for the Department of Work and Pensions added that the government had reduced the proportion of housing benefits spent on private rents since 2012.
“The reality is we have taken action to bring the housing benefit bill under control," they said.
“We are also committed to building the homes this country needs and investing £8bn to build 400,000 more affordable homes.”