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.

The Financial Services Ombudsman 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.

In a letter responding to questions from the Fianna Fáil TD, Michael McGrath, Ger Deering said he is currently dealing with 1,141 complaints related to tracker mortgages.

Of these, he said 103 are being assessed with regard to whether or not they fall outside the statutory time limit, but he expects this number to increase.

Mr Deering added: "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.

"My best estimate at this state is that a time limit assessment will most likely be required on over 400 of those complaints."

Legislation setting out the time limits is so complicated, he said 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."

Mr Deering said that 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 pointed out that other banks are not doing so.

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.

Last month, the central bank completed its final review of the tracker mortgage scandal which found that in total around 40,000 customers had been effected, that banks had paid out just short of 700 million euro in compensation in what it described as distressing and in some cases devastating consequences for customers.

As well as that it found that 99 people had lost their homes as a result of the practice.

Commenting on the latest revelations, Mr McGrath said: "Whatever banks are adopting this approach of invoking the time limit and trying to prevent those cases being dealt with, they really need to change and change that approach immediately."