The software security of high street banks is a concern to many customers and it could be even slacker than we imagined, according to news site Reuters.

A well-known Swedish hacking expert, who did not wish to be named for legal reasons, showed reporters at Reuters how to break through security on web server software from Microsoft used by three of Sweden's biggest banks.

To stay on the right side of the law, the hacker did not enter user's personal accounts, but he showed how an intruder could hide instructions to transfer sums into a separate account when the customer authorises a payment form his or her internet account.

But the Bank of England assured customers that its security parameters had not been breached, although it would not confirm whether it used Microsoft's web server software.

According to Reuters, the Swedish weakness came to light when Microsoft implemented SSL (Secure Socket Layer), an industry standard tool that is used to encrypt transmissions containing private information such as credit card details.

Microsoft admitted there was a flaw in SSL but denied that this posed any widespread threat and dismissed Reuters' claims that it could be used to penetrate bank information.

Whatever the result, this will be a blow to Microsoft which has already come in for some heavy criticism this year over flaws in its web server software. But more than that, it will be a blow to internet banking as whole and spread yet more concerns among users.