A computer worm has forced the shutdown of computer systems at three London hospitals for more than 24 hours, reports BBC News. A hospital spokesman said "well rehearsed" emergency procedures were in place.

The hospitals -- St. Bartholomew's, the Royal London Hospital in Whitechapel and The London Chest Hospital in Bethnal Green -- are part of a single system, Barts and The London NHS Trust. The system went down about noon Monday, affecting Internet access, e-mail and other applications, according to E-Health Insider. By Tuesday morning the system was back up but the worm, described on the BBC site as "not malicious," was more prevalent than ever and a major incident declared.

All but essential systems were shut down to contain it and manual backup systems were used, which in some cases meant writing with pen and paper, according to The Register. Most services were operating normally, including walk-in emergency services. But ambulances were being diverted to other facilities.

The Register reported, however, the culprit was through to be the Mytob worm, which  spreads by e-mail and installs a backdoor Trojan that can give hackers remote control over computers.

The systems are being brought back up, but it could take several days.