Computer systems at three London hospitals have been shut down after being infected with malware.

No patient data was at risk of disclosure, said William Mach, a spokesman for the UK's National Health Service (NHS). As a precaution, computers were shut down at St Bartholomew's, the Royal London Hospital and The London Chest Hospital.

When the infection became known, ambulances were diverted to other hospitals, as it was easier to admit patients using unaffected computer systems rather than revert to a paper-based admission systems, Mach said. The hospitals are now taking emergency patients again, he said.

Official are investigating how the infection occurred, although it did not appear to be malicious, Mach said.

"We've got a team in place that is desperately trying to restore full power to everyone," he said. "They are preoccupied with trying to get the systems online."

The PCs were running McAfee 8.5 anti-virus software, Mach said. That product is also known as VirusScan Enterprise. Mach confirmed that the computers were infected with the mass-mailing worm Mytob, discovered in early 2005.

Once running on a PC, Mytob collects email addresses and mails itself to other PCs as an attachment. Mytob, also known as MyDoom, can also download other harmful software. However, most anti-virus products can detect and remove it. McAfee lists 56 known variants on its website.

Hackers, however, often use a variety of methods to repackage their malicious software using methods such as compression to make a file appear different and catch anti-virus products off guard.

Mach said an internal team has been created to deal with the infections. So far, the NHS has not alerted law enforcement.

"It is not yet known yet how it infected systems in the first place," Mach said.

The National Health Service uses an email system called NHSmail, which contains the contact details for more than one million NHS staffers. According to a NHS website, the system "is protected by cutting-edge anti-virus and anti-spam protection."