Most of the loans bought by Nama are further impaired because of huge contingent liabilities from overcharging by banks, Ireland's leading expert on overcharging has claimed.

Eddie Fitzpatrick, director of Bank Check whose offices in Belfast and Cork uncovered the bulk of overcharging of customers, said the numbers of overcharging cases he has seen point to Nama having overpaid for loans.

Fitzpatrick last week handed voluminous files to Michael Moynihan, chair of the Oireachtas economic and regulatory affairs committee, who is preparing new investigations into the legacy of multi-million "systematic" overcharging by banks that could cost the taxpayer hundreds of millions of euro.

In recent weeks, Anglo has disclosed that it is investigating overcharging of up to €50m on all sorts of accounts up to 2004. Fitzpatrick, who is acting for a number of Anglo customers, said he is certain the final bill facing the public purse will be many multiples of the estimated amount.

But the scale of overcharging by the main lenders at a time when they were free of proper regulation is revealed by the case studies shown to Moynihan last week.

"The enormous scale of the overcharging suggests it went beyond the normal day-to-day errors," Moynihan told the Sunday Tribune. It includes:

* A repayment of €169,000 by Irish Nationwide to a small business customer;

* A repayment of €100,000 by AIB's First Trust Bank;

* A repayment by Ulster Bank of €37,000 to a small business customer;

* A €170,000 repayment by AIB.

Fitzpatrick said Irish Nationwide was in recent days prepared to settle with one of his clients for more than €200,000 but he is claiming the bank actually owes €400,000.

He said he also continues to press several claims against ACC, owned by Dutch lender Rabobank International.

"We have 600 cases of overcharging in ten years," Fitzpatrick said. "I have no doubt the authorities could deal with 600 cases a day because of the scale of the overcharging."

He said in a recent case in Cork he had to challenge a bank that continued to charge interest after the death of a customer: "They said they did not know he had died despite the forms being marked deceased," he said.

A Nama spokesman said the agency "was not liable for any breach of contract, misrepresentation or breach of duty or trust or other legal wrong" in a bank from which it bought loans.