An NHS trust hit by an outbreak of a lethal hospital bug which infected 136 patients and left 20 dead is to be investigated by an NHS watchdog.

The outbreak of Clostridium difficile at the 500-bed Maidstone Hospital in Kent, from April to June this year is one of the worst ever. The Healthcare Commission, which is to carry out the inquiry, said it had come after a previous outbreak last winter and against a "worryingly high background level" of infection.

It is only the second time that the Healthcare Commission has launched an inquiry into the spread of a hospital infection. Last year it investigated Stoke Mandeville Hospital following two outbreaks since 2004.

The commission ordered every NHS hospital in England to review its infection control procedures after opposition MPs warned that the NHS was endangering patients by increasing its work rate and bed occupancy levels.

C. Difficile causes severe diarrhoea which can lead to intestinal gangrene and death and mainly affects elderly patients. The bug forms spores which are difficulty to remove by normal cleaning. Cases have risen from 1,000 a year in the early 1990s to more than 51,000 last year.