Bank customers in Northern Ireland are being ripped off, according to a study by Which?

Customers are charged over 21 times more than their counterparts in Britain and, at the moment, there is little they can do other than switch bank accounts, said Which? - formerly the Consumers' Association.

Presenting its third super-complaint to the Office of Fair Trading (OFT), Which? said the big four banks in Northern Ireland overcharge customers for virtually every service they offer.

The organisation said that, amid mounting concerns over the lack of any real competition in the Northern Irish retail banking market, Which? - working with the General Consumer Council for Northern Ireland (GCC) - was making clear in a message to the OFT that the market simply did not function in the interests of the people it was intended to serve.

Declaring war on the local big four, Mr Phil Evans, principal policy adviser for Which?, said bluntly: "Bank customers in Northern Ireland are being ripped off."

Speaking at a press conference in Belfast, he added: "The big four in Northern Ireland are all offering similarly inferior products, leaving their customers with little choice, indeed a choice between who will rip them off the least."

The targets of the Which? fury are Bank of Ireland, First Trust, Northern Bank (owned by National Australia Bank) and Ulster Bank (owned by the Royal Bank of Scotland, which also owns NatWest).

Which? cited the banks paying "paltry" interest for accounts in credit - generally 0.1 per cent - and charging up to 43p each time a customer used a cash machine, sometimes even when they are not overdrawn.

It added the difference was even more stark when it came to overdrafts.

Which? research showed what it called a "staggering" difference between the cost of running overdrafts with one of the Northern Ireland banks in comparison to one of its "Best Buys".

A customer carrying a £500 authorised overdraft for two weeks per month could pay £11 a year with a Which? Best Buy account, it said.

However, with a current account from one of the big four Northern Ireland banks they could be stung for up to £236.

Mr Steve Costello, GCC chairman, said at the press conference: "In our judgment, the market here plainly does not work for consumers.

"The banks must do the honourable thing by putting their house in order and removing these excessive and unfair charges."