Egg, the online bank, has dumped at least three millionaires as customers because it considered them a bad credit risk. The three, based in London, were among the 161,000 customers whose credit cards were cancelled last month when Egg purged its books of people it feared might fail to pay their bills.

Nigel Griffiths, the MP for Edinburgh South, has asked the Office of Fair Trading (OFT) and the Financial Services Authority (FSA) to investigate the company's actions.

“I think it's unfair for anyone, whether they earn more than a million pounds a year or not, to have their card taken away from them without proper explanation,” Mr Griffiths said.

Yesterday he met Ian Kerr, the chief executive of Egg, to discuss his concerns. The MP said that among the dozens of letters he had received after speaking about the issue on several occasions at least three were from millionaires.

One correspondent said that his gross income last year was £1.1million and that he had worked for one of Britain's 50 largest companies for 26 years.

“My credit rating is excellent and I even happen to have shares worth £180,000 in a Citigroup account,” one wealthy former Egg customer wrote. “I find it outrageous that this bank can make these statements and get away with it.”

Egg, which was set up by Prudential, the UK insurer, was bought by Citigroup, the US banking giant, last May. Egg said that Citigroup immediately began combing through its 2.3 million customer base for bad credit risks.

The bank wrote to 161,000 customers last month to tell them that their cards would be cancelled in 35 days' time because they presented a “higher than acceptable risk profile”.

A spokesman for Egg said yesterday that it did not look solely at credit histories when deciding whether to cut off customers. “Some customers may be up-to-date with payments and have a good record with the credit agencies but our models might show them to pose a higher future risk than we're comfortable with,” the spokesman said.

He said that indicators of credit risk included the number of credit cards that an individual held, whether they were using their credit card more frequently than they had in previous years and whether they used the card to withdraw cash in sums that took them close to their borrowing limit. He did not specify how many of the 161,000 who lost their cards were those that posed a potential risk to the bank.

The spokesman denied that Egg had shed customers who paid off their card every month because they were not profitable. “Profitability is not a factor in our decision,” he said. Egg does not receive interest from these customers but it does receive a fee from every transaction.

Mr Griffiths said that the indicators used by Egg to model future risk would undoubtedly catch some responsible customers as well as those who posed a credit risk. “The whole industry has to look at this since it could unfairly penalise sensible borrowers,” he said.

The OFT said that although credit risk was a matter for individual businesses, it would look at any complaint received and decide whether to investigate. The FSA said that it could not comment on individual companies or investigations.

The action by Egg has been seen as part of a wider trend by banks to move their customers away from risky unsecured lending on credit cards and personal loans and towards lending secured on homes, which is safer for the banks, according to financial website fool.co.uk.

Customers aged between 34 and 49 are most likely to have their credit limits cut or their cards cancelled, the website found, while borrowers younger than 25 who have very little credit history and no property are more likely to have their limits extended.

Some banks expressed glee, albeit privately, at what they saw as a public relations blunder by Egg's new owner. One banker said: “If they thought that this would go unremarked upon they clearly didn't understand British banking”.