Ed Miliband's first act as prime minister would be to release cash to make up the shortfall in benefit payments for tenants hit by the so-called bedroom tax.
In a move that is likely to cost around £100 million, funds would be given to councils to hand over to around 464,000 claimants to top up benefit payments until the "cruel" cut, described as the spare room subsidy under the coalition, is axed.
It would be paid for through the party's wider plans to cover the cost of abolishing the measure by closing tax loopholes.
Mr Miliband made the announcement during a whistlestop tour across Britain that takes in Cardiff, Bristol and Glasgow as he steps up his campaign schedule during the final days of the battle for No 10.
More than 360,000 households in England, 31,000 in Wales and 70,000 in Scotland would be affected by the plans. The policy has been a long-running sore between the Conservatives and Labour.
It deducts money from housing benefit where a household has more bedrooms than residents, with some exemptions.
Under the Labour plans, legislation would be introduced as soon as Parliament is sitting to overturn the reduction in housing benefit.
To cover the cost to tenants while the move becomes law, extra cash would be given to local authorities for discretionary housing payments.
Mr Miliband announced plans to abolish the bedroom tax two years ago, funded by scrapping a scheme where employees give up certain rights for shares, reversing tax cuts for hedge funds, and closing loopholes which allow construction workers to claim falsely to be self-employed.
During a campaign visit to the Millennium Centre in Cardiff, he said: "We will abolish the bedroom tax in every corner of the United Kingdom. This bedroom tax is indefensible. It's cruel, it is unfair and it doesn't even work.
"It's supposed to free up larger properties for families on the housing waiting list but the Government's own figures show that only a handful of tenants have actually downsized their homes.
"It's causing misery for half a million families, it's punishing those most in need in our society, causing untold misery for all of those families. Two-thirds of those families include a disabled person.
"It's a tax that affects 200,000 children and 60,000 carers across Britain - people we should be helping, not hurting.
"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."
He added: "It tells you everything you need to know about this Tory Government and it tells you everything you need to know about a Labour government that the first thing we would do is scrap the bedroom tax.
"We will legislate straightaway to abolish the bedroom tax - in England, in Wales, in Scotland, right across our United Kingdom.
"We'll get to work immediately to ensure that families no longer lose out. We'll make new funds available to local authorities to offset the full costs of the tax for all families who currently pay it.
"So on day one of a Labour government, we free families from the burden of the bedroom tax."
Labour unveiled a new campaign poster claiming Conservative plans would leave 4.3 million families £1,000 a year worse off through cuts to child benefit.
It follows Liberal Democrat revelations of proposals discussed under the coalition for £8 billion cuts to the welfare payment, including introducing means-testing, removing it from 16-to-19-year-olds, and limiting the benefit and child tax credit to two children per family.
Treasury Chief Secretary Danny Alexander suggested it gave an indication of Tory plans after the May 7 election but Mr Cameron said last night he "rejected at the time as Prime Minister and I reject again today".
Mr Miliband said: "Now we know the Tories are planning to devastate family finances.
"For those of you who saw Question Time last night, the Prime Minister was asked three times whether he planned to cut child benefit.
"He apparently used 600 words when just one word would have done, the word 'no'."
During the visit, Welsh First Minister Carwyn Jones said voters faced a "stark choice" at the ballot box next week.
He said: "The choice is between a Tory future, a future where there isn't a war on poverty but a war on the poor. A future where people work ever harder for less money.
"And then there's the Labour future, a future where people get a fair day's pay for a fair day's work."
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