The Financial Services Ombudsman, Ger Deering, has raised concerns that some banks are contesting his office's attempts to investigate complaints, claiming they fall outside a legal six-year time limit.
Hundreds of borrowers who believe they were affected by the tracker mortgage scandal may not have their complaints heard because their lenders say they have left it too late.
Mr Deering told RTÉ radio’s Morning Ireland that he is currently dealing with 1,141 complaints related to tracker mortgages.
Of these, 103 are being assessed with regard to whether or not they fall outside the statutory time limit, but he expects this number to increase.
"Based on our current experience, I believe that as we progress additional complaints to investigation there will be more complaints where the statutory time limits will be an issue.
But there could be other high-risk cases outside the time limits, he warned.
There is also the issue of whether the time limit is six years from when the incident occurred or from the “date of knowledge” when the mortgage holder became aware of the change.
Legislation setting out the time limits is so complicated, Mr Deering said in a letter responding to a question from Fianna Fáil’s finance spokesperson Michael McGrath, that "this office in some instances spends at least as much time dealing with assessments relating to time limits as it would in conducting a full formal investigation of the merits of that same complaint."
Some banks are "rigorously challenging the jurisdiction of this office to deal with complaints where there is a question in relation to whether the complaint was made outside the time limits," he said.
This is despite an understanding in a framework set up between the Central Bank and the lending institutions that they would not raise the statute of limitation defence.
In its final review of the tracker mortgage scandal, published last month, the central bank found that in total 40,000 customers had been affected and that banks had paid out almost €700m in compensation in what it described as “distressing” and in some cases “devastating consequences” for customers.
It also found that 99 people had lost their homes as a result of the practice.