Housing benefit claimants are to begin receiving cuts to their payouts from today, as the coalition implements its "wicked bedroom tax".
New under-occupancy rules mean claimants with spare bedrooms will be punished by getting less of their rent paid for by housing benefit.
A tenant with one spare bedroom will lose 14% of their housing benefit, while a property with two spare bedrooms gets a 25% cut.
"This wicked bedroom tax is going to rip neighbour from neighbour, force vulnerable people to food banks and loan sharks, and end up costing Britain more than it saves as tenants are forced to go homeless or move into the expensive private rented sector," shadow work and pensions secretary Liam Byrne said.
"It is the worst possible blend of cruelty and incompetence. The government must think again and drop this tax now."
Labour has calculated 19 out of 20 families affected by the reform will not receive any assistance from their local councils.
Chancellor George Osborne and work and pensions secretary Iain Duncan Smith defended the reforms in an article for the Telegraph newspaper, however.
"Today, more than one million people in England are living in overcrowded housing," they argued.
"And millions more are stuck on social-housing waiting lists, desperate for the opportunity to be given a home for their families. This is a scandal. To address this problem we need to use our social housing more fairly and effectively. That is why we are ending the spare-room subsidy from today."
The changes will not apply to pensioners, those living in shared ownership properties or people living in some types of supported accommodation.
Providing advice to those affected, housing charity Shelter suggested: "It might be possible to make some savings here and there, or there might be state benefits and tax credits that you're entitled to but not claiming at the moment.
"If there's no way you can manage you'll have to think about moving to a smaller home in the local area, or even moving out of the area altogether if there's nothing suitable for you locally."
Shelter found last week that four out of ten families with children had cut down on their food costs to pay for their housing costs in the last year.
"It is wrong to leave people out in the cold with effectively no roof over their heads because the taxpayer is paying for rooms which aren't in use," Conservative party co-chair Grant Shapps told Sky News.
"It's just a common-sense reform which in the end will help house more people. People share rooms quite commonly - my boys share a room."
Osborne and Duncan Smith added: "Of course, if you listened to the shrill voices of the left you'd think that every change to the welfare system, and any attempt to save money, marks the beginning of the end of the world," the pair wrote.
"In reality, we are just restoring the original principles of the welfare state: that those who can work must work, and a life on benefits must not be more attractive than working."
The remainder of the coalition's welfare reforms will be rolled out over the next two weeks. April 15th sees the introduction of the benefit cap, in which households receiving out-of-work benefits will no longer receive more than the average weekly wage.