Official figures for mortgage arrears collected by the financial regulator seriously understate the scale of the mortgage-debt crisis here by potentially billions of euro, mortgage brokers and debt counsellors have warned.

The Sunday Tribune has learned that the regulator is preparing to publish its new survey of arrears which will likely show only a modest rise in the 26,271 mortgage accounts, worth €4.8bn, which the banks said were in arrears last September.

But the figures will not include more than 30,000 mortgage accounts which have switched to interest-only or renegotiated their mortgage terms because the holders were in danger of skipping monthly payments, the banks have confirmed.

Bank of Ireland, AIB and Permanent TSB said they do not include customers who have renegotiated their mortgages or switched to interest-only payments in compiling their arrears figures.

"Anybody who switched to interest-only payments during this time when rates are at the lowest we have ever known is facing stress in meeting payments," said Michael Dowling, former president of the Independent Mortgage Advisers Federation. Dowling said households were struggling to service as much as €8bn in mortgage debt.

A spokeswoman for the financial regulator said its quarterly survey was based on arrears data provided by 22 mortgage lenders. "Data on types of mortgages or those in negative equity was not requested as our focus is on arrears and repossessions," she said.