A quarter of households hit by the 'bedroom tax' have been pushed into rent arrears for the first time, just five-and-a-half months after it was introduced.
National Housing Federation chief executive David Orr said the figures underlined demands for the housing benefit cut to be scrapped to prevent families entering a "spiralling cycle of debt".
He added: "What more proof do politicians need that the bedroom tax is an unfair, ill-planned disaster that is hurting our poorest families? There is no other option but to repeal."
Under the Government's welfare reform, social tenants deemed to have more bedrooms than they need have had their housing benefit reduced.
Ministers say private sector renters do not get spare rooms for free, and argue the change will save around £500 million annually.
It has sparked protests across the country with opponents claiming it is forcing families into poverty and will increase the benefits bill by pushing people into the private sector.
The Federation found that a quarter of tenants affected by the reform in 38 housing associations it questioned went into arrears for the first time between April and June.
Just over half (51%) of the 63,578 tenants of 51 housing associations were unable to meet their rent payments in the first months of the new system.
United Nations special rapporteur on housing Raquel Rolnik has called for a rethink after finding the reform was causing "great stress and anxiety" to "very vulnerable" people.
Her intervention was met with fury by Tory chairman Grant Shapps, who wrote to UN secretary general Ban Ki Moon demanding an apology and an explanation for the "disgraceful" comments.
Liberal Democrat activists voted overwhelmingly at the party's conference in Glasgow to commit the party to a review of the policy's impact on vulnerable families - but Nick Clegg was left tongue died when challenged by a disabled women over it.
Orr - who will set out his criticisms in a speech to the Federation's conference tomorrow - said: "This is the most damning evidence yet to show that the bedroom tax is pushing thousands of families into a spiralling cycle of debt.
"If these figures are replicated nationwide, over 330,000 households could already be struggling to pay their rent and facing a frightening and uncertain future.
"What's more, people can't even move to smaller homes to avoid the bedroom tax because there aren't enough smaller properties out there.
"Housing associations are working flat-out to help their tenants cope with the changes, but they can't magic one-bedroom houses out of thin air. People are trapped."
Sovereign Housing Association said most of its 2,360 households affected by the benefit cut fell into arrears and a third remained behind with the rent.
Chief executive Ann Santry said: "With huge demand for affordable homes we recognise the need to maximise the use of every room and so we're investing heavily in supporting our residents to adapt to the changes wherever possible.
"However, six months in, we are seeing the most vulnerable families, couples or individuals on the lowest incomes beginning to struggle.
"Sometimes there are simply no suitable smaller properties available while in other cases the cost of moving a disabled resident from their two-bed home to another property which will need to be adapted seems to outweigh any benefits to the public purse.
"The policy also takes no account of the size or suitability of 'spare' rooms. Is it reasonable to expect two teenagers to share a tiny box room, or a 15-year-old to share with her baby sister?"
Shadow work and pensions secretary Liam Byrne said: "The jury is now in.
"David Cameron's hated 'bedroom tax' is pushing a generation into foodbanks and loan-sharks. This Government seems determined to stand up for a privileged few, but stands idle while hundreds of thousands of our neighbours are pushed into debt from which they may never recover."
A Department for Work and Pensions spokesman said: "The removal of the spare room subsidy is a necessary reform to return fairness to housing benefit.
"Even after the reform we pay over 80% of most claimants' housing benefit - but the taxpayer can no longer afford to pay for people to live in properties larger than they need. It is right that people contribute to these costs, just as private renters do.
"It is just wrong to suggest the early stages of the policy - as people start to adjust to the changes - represent long-term trends in any way whatsoever or that a self-selecting poll of a small minority of landlords provides a clear picture of the reform."
The Government has made available £190 million of extra funds to support vulnerable tenants as the policy takes effect and is monitoring its use.