Some of the worst-off social housing tenants will have to pay shortfalls of up to quarter of their rent if fresh welfare proposals are introduced, the Federation of Northern Ireland Housing Associations has warned.
Huge hardships face those relying on benefits if a "bedroom tax" inspired by Westminster is brought into Northern Ireland, the organisation added.
A penalty could be applied to payments for tenants who occupy more bedrooms than they need to encourage them to move into smaller properties under draft legislation.
It is part of the Welfare Reform Bill to be considered in the Assembly on Tuesday - although Sinn Fein has called for it to be deferred because of its impact on the most vulnerable.
Cameron Watt, chief executive of the Federation, said: "We have great fears that the bedroom tax is going to hugely hurt social tenants in Northern Ireland."
He added: "Social tenants in Northern Ireland will be disproportionately affected by this policy because in Northern Ireland the majority of social housing stock is family-sized accommodation and there is a real shortage of one and two bedroom social homes."
Key features of the Welfare Reform Bill include a universal credit to cover a range of benefits, a personal independence payment reassessed every three years to replace Disability Living Allowance and housing benefit reforms.
At present, money comes from Westminster via the Department for Social Development (DSD) to housing associations. DSD is proposing withholding some of the money paid on behalf of social housing tenants from the associations and Housing Executive, with a knock on effect on tenants.
Mr Watt said huge numbers of tenants with major shortfalls in their benefits will have to find quite significant sums of money, up to 25% of their rent each week, from very limited benefits.
"It is going to cause huge hardship for tenants but it will also cause major problems for landlords (like the Housing Executive and housing associations) who will face a big increase in arrears affecting their ability to build new social housing," he added.