About 33,000 benefit claimants in Northern Ireland could lose out once special exemptions end, the Housing Executive warned.
Measures designed to limit the impact of welfare changes are due to lapse in 2020 and no ministers are in place at Stormont to engineer their renewal.
A total of £21 million could be lost each year.
Sandra Moore, chief executive of the Welcome Centre which works with the homeless, said those who lived on the breadline would immediately be placed at risk.
She warned: “The Housing Executive are facing a black hole.”
The Executive’s own research noted potential difficulties for tenants and landlords if mitigation ended as scheduled due to the reduction in housing benefit or Universal Credit (as a result of under-occupancy) for around 33,000 existing social sector claimants in Northern Ireland.
It added: “If mitigation and welfare supplementary payments end as scheduled, tenants in Northern Ireland collectively stand to lose over £21 million in benefits each year.”
33,000 social housing users threatened by welfare mitigation changes from 2020.
£21 million worth of benefits could be lost each year.
30% of Housing Executive tenants are affected by the bedroom tax.
One in ten in Northern Ireland are on Disability Living Allowance.
Discretionary payments could be used to help those struggling to make up housing costs shortfall, research from the public housing body suggested.
Civil servants have been given greater decision-making powers by the Northern Ireland Secretary.
Key elements of welfare reform were introduced to Northern Ireland in 2016, including the freezing of local housing allowance levels, application of a benefit cap, and changes to housing benefit.
A social sector size criteria, which critics term a bedroom tax, was then introduced in February 2017 and means reduced benefit if a house is considered under-occupied.
The Executive’s report said: “The waiting period for Universal Credit will likely result in increased rent arrears and referrals to food bank services are likely to increase; as highlighted by The Trussell Trust reporting, there has been a 30% average increase in three day emergency food supplies in the months after the introduction of Universal Credit.”
The potential impact of the full introduction of Universal Credit was eased by direct payments to landlords, twice-monthly payments to claimants, greater support for claimants through the funding of the advice sector, some of which will apply to its operation even after other welfare reform mitigations end.
The Housing Executive added: “Whilst we welcome the amendments and nuances that have helped Northern Ireland cope with changes, there are inevitable concerns about the impact of welfare reform if the mitigation budget expires as planned in 2020.”
Approximately 30% of its tenants and 25% of housing associations’ tenants on housing benefit will be affected by the bedroom tax, the Executive said.
The housing body added: “If appropriate legislation was in place, there is potential to extend the use of discretionary housing payments on a wider basis in Northern Ireland (as already occurring in Great Britain) to help mitigate the impacts of the loss of welfare supplementary payments for those struggling to make up their housing costs shortfall.
“Due to a lack of available smaller dwellings within Northern Ireland’s social housing stock, many tenants would be unable to move to a property with fewer bedrooms – even if they are willing to do so.”