Three hospitals in London were forced to shut down their networks Tuesday after being infected with a computer virus.

"Emergency procedures have been activated to ensure that key clinical systems continue while network access is being established. We have maintained a safe environment for our patients throughout the incident," a statement on the site for Barts and The London hospital system said.

"Manual backup systems are in use and we are in the process of restoring the computer systems with priority being given to the most important areas for maintaining patients services," the statement said.

The hospitals affected are St. Bartholomew's, the Royal London Hospital and The London Chest Hospital. The BBC quotes a spokesman for Barts and The London as saying the virus was "not malicious" and the infection was "self-contained."

Doctors are using pen and paper as backups as a result of the infection, according to The Register.

The virus is believed to be the Mytob worm, which spreads via e-mail and plants a backdoor Trojan on infected computers that can be used to remotely take control of the machine, according to security firm Sophos.

"There will, no doubt, be concerns that the confidentiality of patients' data may have been put at risk and the hospitals will surely be keen to reassure the public that security has been maintained," Graham Cluley, senior technology consultant at Sophos, writes in a post on his blog.