The Comptroller and Auditor General is expected to express concern in a report to be published shortly at the "rapid increase" in the cost of rent supplement, which has grown from €150 million in 2000 to almost €400 million.
Rent supplement is aimed at providing short-term income support to people on low incomes who cannot afford to live in private rented accommodation.
The number of households in receipt of the rent supplement has now reached almost 60,000, compared to 30,000 in the early 1990s when the scheme was established. Rent supplement recipients now account for about one-third of the private rented sector in the State.
The C&AG's value-for-money report will consider whether the Department of Social and Family Affairs managed the scheme with "due regard for economy", question its "operational controls" and assess the payments in the context of social housing provision.
A spokesman for the Minister for Social Affairs, Seamus Brennan, said the department received the report last month and it would be published shortly.
The widespread use of rent supplement for long periods has led to a number of schemes to try to reduce the numbers in receipt of the benefit. However, they have not met with much success.
Under the rental accommodation scheme, announced in July 2004, local authorities became responsible for providing tenants in receipt of rent supplement for 18 months or longer with long-term housing.
However, figures compiled by the Department of the Environment for late last year showed that 31 tenants, or 0.1 per cent, have been provided with such long-term housing.
Meanwhile, Mr Brennan is understood to be investigating if the refusal by landlords to accept rent supplement is in breach of the law. He is understood to have expressed concern that such moves may be undermining attempts to create a social mix in private rented accommodation.
The Department of Environment, meanwhile, said it anticipated that all tenants in receipt of rent allowance for more than 18 months would be automatically provided with long-term accommodation from September 2008.
However, the slow pace at which the scheme is operating appears to put this in doubt.
It said the new scheme was still being rolled out, and local authorities were in the process of negotiating with landlords to form a stock of contracted accommodation, as well as negotiating with existing landlords of tenants on rent supplement.
It said it was possible that local authorities may not be able to provide a suitable accommodation arrangement in particular cases, either through lack of immediate availability or due to some problem with the person's existing accommodation.
In the event of suitable accommodation not being available for a person who has been on rent supplement for 18 months or more, there was no question of withdrawing the benefit.