A Labour government would scrap the so called "bedroom tax" on their first day in office after the general election, Ed Miliband will promise.
The Labour leader is expected to say that the "cruel and unfair" spare-room subsidy has hit almost half a million people and the first act of his administration would be to axe it.
"We're going to abolish the bedroom tax in every corner of the United Kingdom. This bedroom tax is indefensible. It's cruel and it doesn't even work," Miliband will say.
"It has punished those most in need in our society, causing untold misery for half a million families across the UK, two thirds of which include a disabled person."
"So on day one of a Labour government, we free families from the burden of the bedroom tax."
David Cameron's government introduced the "bedroom tax" in a bid to free up properties and help tackle the country's lack of affordable housing.
The subsidy takes a quarter of housing benefit away from claimants, who rent from councils are housing associations, if they are deemed to have exceed the spare-room limit.
"It's a tax that affects 200,000 children and 60,000 carers - people we should be helping, not hurting," Miliband will say.
"And under the Tories, a million more people could be hit by the tax in the next five years. This is a Government that cuts taxes for millionaires, opposes the mansion tax. It won't abolish non-dom status but will keep the bedroom tax.
"I think that tells you everything you need to know about a Tory government - and it tells you everything you need to know about a Labour government that the first thing we'll do is scrap it."
The pledge will come with just six days to go before the general election, with Labour one point ahead of the Tories (35% vs 34%).
The survey, conducted between 29 and 30 April, also put Ukip on 12%, the Liberal Democrats on 8% and the Greens on 5%.