Bank customers have been warned that money mistakenly credited to their accounts is likely to be taken back.

Around 100 new cases of so-called "mis-applied credit" are brought to the Financial Ombudsman Service every year, with very few rulings ending in the customers favour.

In the first 3 months of 2010 the ombudsman received 39,576 new complaints regarding all financial services.

The ombudsman service offers free assistance to help settle complaints between consumers and financial businesses on anything from pawnbroking to mortgages.

Mis-applied credit falls under complaints about current accounts made to the ombudsman.

Throughout the 2009-10 financial year there were 24,515 cases filed under current accounts. There have been an additional 5,420 complaints in the first quarter of this financial year.

Many people raise complaints about banks chasing back money that was mistakenly added to their bank accounts.

Some customers argue that the error was no fault of their own so they should keep the money. The ombudsman, however, very rarely agrees with this.

"In the board game Monopoly, it is good news if you get a card telling you that the bank has made an error in your favour - as you do get to keep the money," said chief Financial Ombudsman Natalie Ceeney in Ombudsman News.

"But in real life, things are different.

"When dealing with complaints about mis-applied credit, we generally take the view that consumers are required to return any money paid to them by mistake."

One example was when a customer transferred £1,000 into his bank account, but the bank accidentally duplicated the transfer and added another £1,000 into the account.

When the bank asked for the extra £1,000 to be repaid two weeks later, the customer refused, saying that it was the bank's mistake and that he had already spent the money.

The ombudsman ruled that the customer must pay back the money, but allowed the customer to make 10 interest-free instalments.

However, in a separate case, the ombudsman decided that a customer could keep the extra cash.

Her account was credited with £1,000 in a similar mistake after she paid in a cheque of just £100. After the customer noticed the error after checking her balance on a cash machine, she spoke to a cashier inside a branch, who confirmed it was in fact correct.

The recently-retired customer had been expecting a tax rebate and assumed this was where the extra cash came from. Soon after she took her niece on a holiday to Italy using the money.

Almost four months past, after which she received a letter from her bank demanding £900.

The ombudsman ruled that she acted in good faith after checking with the cashier and was unable to recover any of the money spent on the holiday. She was therefore not made to repay any of the money.