UNDECLARED top-ups are being paid to landlords by a majority of people on rent supplement, a new report is expected to show.
More than 55pc of recipients pay additional sums in rent over and above approved amounts, according to research by Threshold.
This has left many low-income families struggling to afford the bare necessities on a weekly basis, the national housing charity said.
Threshold chairwoman Aideen Hayden told the Herald that the figure is "55pc-plus" in terms of the numbers who are topping up.
The allowance is paid by the Department of Social Protection to tenants in private accommodation who cannot afford their rent. It is only provided if the rent is below a maximum limit, which varies by county.
Recipients are required to make a contribution, but it is capped to ensure they have enough money to live on.
However, the payment of additional under-the-counter sums means families are depriving themselves of other essentials, said Ms Hayden, Labour's housing spokesperson in the Seanad.
"Say the maximum you are allowed to pay (in monthly rent) for a property in an area is €950 for a single parent with two children and the cheapest you can find is for €1,200. You can't pay €1,200 because it's above the rent cap," she said.
"The problem is the rent cap will not get you somewhere to live, so they pay the difference between €950 and €1,200 under the counter.
"The problem with that is that people get themselves into debt and deprive themselves of the basics."
It is recognised that if tenants are allowed to make too high a contribution, "it then puts people at risk of extreme poverty", Ms Hayden said.
Many of the top-ups are being paid in respect of properties "which are not of particular quality", she added.
The senator has told Social Protection Minister Joan Burton that the rent limits in "parts of Dublin are not high enough to get somewhere to live".
The Department of Social Protection has not responded to her comments.