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More than 330,000 children will be hit by Tory plans to slash the benefit cap - and nearly two thirds of the victims will be single mums.

The devastating toll has been unveiled in the government's own impact assessment into how the policy will work.

Documents slipped out during last night's welfare debate - but not published online thanks to 'logistical issues' - show 126,000 families will lose £63 a week.

The Chancellor is also lining up a £20bn raid on public spending to be unveiled in just four months.

And the revelations came as 48 Labour MPs defied their acting leader to try and block Conservative welfare cuts.

Harriet Harman had urged backbenchers not to oppose the latest social security assault, including the benefit cap, in a bid to show Labour had "listened" to voters' devastating general election verdict.

How many homes are affected by the benefits cap? Source: DWP impact assessment

The figures slipped out as MPs debated the Conservatives' Welfare Reform and Work Bill showed the huge effect fresh cuts would have on families with the lowest incomes.

As last night's fiery Commons exchanges got underway, the Department for Work and Pensions quietly published the Welfare Reform and Work Bill impact assessment.

It reveals that slashing from £26,000 the maximum benefits a family can claim will plunder £63 a week from average budgets of those affected - £3,276 a year.

Single mums will by far be hit hardest by the move to a £20,000 limit, or £23,000 in London.

A whopping 64% of all affected claimants are expected to be single women compared to just 12% for single men.

The vast majority of the women - 59% of all those affected - will be single mums.

In total 92,000 new households will be drawn into the cap on top of the 34,000 who would already be affected.

How much will benefits cap raid from families? Source: DWP impact assessment

The worst-hit will be those who are already capped - losing £64 a week on top of what they've already suffered. Those brought into the cap for the first time will lose £39 a week on average.

It'll mean picking £95m from the pockets of all those affected next year, soaring to £300m by the end of 2017/18. By 2021 it'll be £480m.

The assessment claims the policy will deliver "further positive change" by encouraging cash-strapped families into work.

But it admits a whopping £800m will have to be set aside for 'discretionary housing payments', which cover desperate benefit claimants who can't afford their rent, by 2020.

Spending on the payments has already soared thanks to the bedroom tax.

A DWP spokesman said: "The benefit cap provides a clear incentive for people to move into work and ensures we have a welfare system that is fair for those who need it and those who pay for it.

"We know that many people who have had their benefits capped move into work, reduce their housing benefit claim or are no longer claiming housing benefit at all, and these figures do not take into account this behavioural change."