"What is important is that none of our customers will have lost their money," said a bank spokesman.

"We are doing all we can to stop this."

Largest Nordic bank

The attack used a program called a trojan, known as haxdoor.ki, to obtain customers details. Trojans are programs that look benign but contain malicious software.

Victims were duped into downloading the program after receiving an email, purporting to come from the bank, encouraging them to download anti-spam software.

Once installed the trojan monitored the PCs' online activities.

"It listens for key web addresses, in this case the Nordea bank address," said Greg Day, security analyst at McAfee.

When a user navigated to the Nordea bank login page, the trojan would kick into action, saving the customers login details. It then displayed an error message asking them to resend the information.

With two access codes the criminals could transfer money from the customer's accounts.

Trade newspaper Computer Sweden said the police had traced the fraudulent emails first to computer servers in the US and then to Russia.

Nordea added that customers who had been affected often did not have any anti-virus protection on their computers.

The bank, which has 2.2 million customers, is the largest across the four Nordic nations of Norway, Denmark, Sweden and Finland.

In August 2005, it was forced to temporarily shut down its online arm due to a sophisticated phishing attack.