Regulatory officials are investigating about 20 instances of overcharging at a "small number" of financial institutions, the Central Bank has confirmed.

The news comes as the Central Bank is pushing for tougher rules, and penalties, governing the way regulated firms have to review, monitor and rectify errors. Under the proposals, banks and other financial firms will have to resolve mistakes and compensate affected customers within six months of discovering discrepancies, or face fines.

Last month, regulators hit AIB with a €2m fine – the biggest ever for a retail bank – for its poor handling of overcharging errors stretching back years.

Head of financial regulation Matthew Elderfield is reportedly seeking authorisation to double the maximum fine from €5m to €10m.

Last year Elderfield began setting deadlines on a backlog of overcharging cases involving banks and insurers that had identified problems but failed to act on them. Officials threatened them with fines, naming and shaming and other administrative sanctions if they did not notify customers immediately and act quickly to fix the problems.

The Central Bank levied €2.25m in fines last year, compared to €3.67m in 2009 and €3.7m in 2008. According to Central Bank records, the financial services industry repaid €180m to overcharged customers in the past decade, including €88m repaid so far by AIB alone.

A spokesman said the Central Bank could not comment on which institutions were under investigation. He said some had committed simple errors while others had systemic problems.

Under new rules, regulated firms will have to test their internal controls regularly to identify and minimise charging errors. Firms will also have to devise procedures for handling errors when they occur. Banks and other financial institutions will also have to inform the Central Bank in writing of any overcharging and disclose how many consumers were affected and how much they were owed. Firms are to be barred from deriving any benefit from refunds which cannot be repaid.

The Central Bank has identified priority areas for enforcement, including overcharging. Accounting, auditing and other internal controls are set for fresh scrutiny as well, according to a spokesman.