More than 8,000 computer viruses have disrupted NHS IT systems in the past year, an investigation by More4 News has revealed.
The first national figures on computer virus infections at NHS trusts emerged after 75% responded to a freedom of information (FOI) request.
More4 requested the information after the Mytob worm brought down computer networks and systems at three London hospitals in November 2008.
The disruptions prevented access to X-rays and other patient information at Barts, the Royal London and the London Chest hospital. They also caused delays in providing care and treatment.
The FOI request revealed similar disruptions at more than ten hospitals throughout the country.
Several NHS trusts reported computer systems were locked or frozen. Critical appointments were either rescheduled or cancelled as a result of computer viruses.
A number of trusts admitted their networks were attacked because anti-virus systems were turned off or not properly applied. Others blamed staff for turning off automatic updates.
An independent review of the Mytob outbreak at the three hospitals in London concluded that the incident was entirely avoidable.
There was a substantive failure of the Trust's information governance processes, "especially those operational processes in the ICT [information and communication technologies] domain", the review said.
Failure to stop viruses from infecting NHS computer systems not only risks delays to vital services, but also increases the risk of patients' personal data being lost or stolen.
This has raised concerns in the light of the government's plan to build a £12.7bn system to link the medical records of everyone in England.
The NHS said in a statement that electronic record systems are protected by the highest levels of access controls and other security measures.
"These levels of security are far higher than any which can be imposed on access to paper records or the majority of local NHS IT solutions," the statement said.