The under occupation penalty - known as the bedroom tax - comes into effect on 1st April.
The people most likely to be affected are singles, couples in two and three-bedroom homes and people whose children have left home.
From April only one bedroom will be allowed for each adult or couple.
This will mean there will be a reduction in the housing benefit paid to tenants of working age.
Tenants under-occupying by one bedroom will have a 14 per cent reduction. Those under occupying by two bedrooms or more will have a 25 per cent reduction.
The tenants affected will lose on average £11 per week.