Customers of a Swedish bank have had almost €1 million removed from their accounts by Russian hackers in the biggest internet fraud to date.

Over more than a year, some Nordea Bank customers received e-mails, purporting to be from the bank to help them combat unwanted e-mails. However, instead of protecting them, the special programme launched a virus passing on their details to criminals.

It is believed that a wide conspiracy involving more than 120 Russians from organised crime gangs were behind the attack.

The programme then invited the customers of Nordea bank to input confidential information, which was then transmitted back to Russia via the US - a scam known as "phishing".

The bank, with more than two million customers across Scandinavia, has refunded all affected customers. It believes that most of the victims had not been running anti-virus programmes on their home computers, which might have identified the virus.

After users logged in to the bank's website, a false error message appeared requesting customers to key in password details again, which then left their computers open to attack.

Since the fraudsters avoided withdrawing large sums, the theft was not immediately obvious and continued for 15 months.

Although scams have been organised by hackers in many countries, numerous computer viruses and online fraud are believed to have originated in Russia.