NHS computer systems were infected with more than 8,000 dangerous viruses last year, an investigation has found.

The investigation by More4 news highlighted computer technology glitches at a number of NHS trusts in the financial year 2007-08 and said most could have been avoided had the trust simply updated its anti-virus software.

Trusts exposed by the investigation include Sheffield Teaching Hospitals foundation trust. Eight hundred of the trust’s 7,000 personal computers were infected with the Conficker B virus causing some of its non urgent appointments to be cancelled.

The trust’s informatics director David Whitham said: “We cannot be certain how the virus entered the network but at around the same time as the virus became evident the automatic Microsoft update process had been temporarily disabled following problems with some PCs providing supporting information in theatres.”

Last year, a Mytob worm virus outbreak at Barts and the London NHS trust led to patient appointments being cancelled as staff had to resort to manual systems. An independent report described the incident as “entirely avoidable” as anti-virus software had been configured incorrectly.

The revelation that NHS trusts have been poor at keeping their anti-virus software up to date has provoked concerns that they are vulnerable to viruses that could cause confidential patient data to be disseminated.

But a spokesman for the Department of Health said the electronic patient records systems provided through the national programme for IT were “protected by the highest levels of access controls and other security measures”.

He added that local NHS trusts were legally responsible for complying with data protection rules and were expected to record any breaches.