We've heard a lot about the Government's headline-grabbing benefits cap, which threatens to plunge 50,000 larger families into poverty. But a separate and lesser known plan to punish social tenants for 'under-occupying' their homes is just as unfair and will hit more than 10 times that number of low-income people.

The Government has admitted its proposed 'size criteria', which forms part of the Welfare Reform Bill, will cost 670,000 tenants an average of £676 per year. That includes families who are deemed to have just one 'spare' bedroom, perhaps because their children have left home or a family member has died. Two thirds of those affected are disabled, the Department for Work and Pensions has admitted.

However, for households deemed to be under-occupying by two or more bedrooms, the 25% reduction in housing benefit being contemplated could cost families living in three-bedroom homes in London, for example, an eye-watering £1,385 per year.

And in the North West - where almost half of all working-age housing benefit claimants in the social sector face being hit by the penalty - families in similarly sized properties face losing up to £955 a year.

This bedroom tax will have disastrous implications for a huge number of people already struggling to make ends meet in this tough economic climate, including grandparents, disabled people and smaller families.

It's important to recognize that this blunt measure is not simply taking aim at families with bedrooms lying empty. For example, under the Government's restrictive new rules, same sex teenagers up to the age of 15 will be forced to share a bedroom. And foster parents will be affected even where their bedrooms are occupied by foster children, who for benefit purposes do not count as part of the household.

Others threatened with benefit cuts include lone parents or grandparents who use their 'spare' bedroom to share the care of their children or grandchildren, couples who sleep separately for medical reasons and disabled people who have had their homes specially adapted for their needs.

Crucially, that extra room often isn't 'spare'. It is the place a family carer stays when a parent is ill, or the space a teenage child needs for privacy and study. It is part of normal life. We believe that penalising some families for living the lives most people lead is unfair and unjust. Yet that is what is going to happen from April 2013 if the Welfare Reform Bill goes through unamended.

Of course, everyone understands the desire to make best use of our inadequate supply of affordable homes. But these families will be forced to choose between going into debt, struggling to meet payments by cutting back on essentials, or trying to move - even if no suitable alternative properties are available.

Ahead of the Bill's Report Stage in the House of Lords, which begins today, the National Housing Federation is calling on the Government to make the rules more flexible, to allow one additional bedroom above that permitted by the proposed criteria.

Crossbench peer Lord Best has tabled an amendment to this effect and we ask members of the House of Lords to support it.