David Cameron has promised to review "any individual case" relating to the so-called 'bedroom tax'.
Legal action will be launched against the plans today, which could see housing benefit payments reduced for tenants in council or housing association properties with empty bedrooms.
Under the new rules, all working-age tenants in receipt of housing benefit will be affected by the charge - 14 per cent for one unoccupied room, 25 per cent for two.
The charge would apply, irrespective of whether the room is occupied on a part time basis, such as a couple who use their spare bedroom if one of them is recovering from an illness or operation.
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Of the 1,600 business leaders questioned, a third planned to scale back hiring staff if the £7.20 an hour rate for adults increases.