The number of severe faults in NHS computer systems has almost doubled over the past three years.

Last year, nationwide NHS computer systems suffered 820 "severity one" or critical faults with national applications, compared to 488 in 2006.

A severity-one fault is a problem affecting a system critical to patient care or affecting 5,000 NHS computer users or more.

The figures were revealed in a written parliamentary answer to Liberal Democrat MP Norman Lamb.

In October 2008 the number of critical faults in national IT systems jumped to 165 from 71 the previous month due to problems with two particular systems.

NHS Connecting for Health said the rise was because more systems were being used across the NHS after the £12.7bn National Programme for IT (NPfIT) rollout, which has suffered some technical problems and parts of which are four years late.

"Comparing the number of reported incidents in 2008 against 2006 in this way is meaningless, as the rise is the result of the deployment of more systems under the NPfIT and active encouragement to report incidents," said a Connecting for Health spokeswoman.

"The fact that no incidents resulted in harm to patients shows how effective NPfIT is as an international leader in standards of safety for the manufacture and use of clinical systems."