Professor Brian Duerden, deputy director of the Public Health Laboratory Service (PHLS), says incidents involving MRSA - methicillin-resistant staphylococcus aureus - have risen 12-fold since 1991 and are responsible for 37% of fatal cases of blood poisoning, compared with 3% eight years ago.

Two particularly strong strains of the bug are thought to be behind the rise in cases.

The PHLS has warned hospitals to tighten their measures for controlling infections, such as basic hygiene rules, and GPs have been warned to reduce prescription of antibiotics to avoid people developing resistance to them.

Wales and the West Midlands have experienced the greatest increase in cases.

The PHLS says incidents in the West Midlands have risen three-fold since 1995 when there were 146 cases. Some 427 cases were reported in 1997.

Wales has had an eight-fold increase in the same period with 48 cases in 1995 and 393 cases in 1997.

Common bug

Staphlococcus aureus bug is carried by a third of the population and is one of the most common bacteria, causing boils, skin problems and septicaemia.

However, many people carry the bacteria without ever displaying symptoms - these generally only arise in certain patients whose health is vulnerable during serious illness or following surgery.

Hospital patients and elderly people in nursing homes are particularly at risk of infection because they often have exposed wounds and a weakened immune system.

The problem with MRSA, which is carried on the skin and mucous membranes, such as the nose, is that it is resistant to many antibiotics.

This reduces the treatment options available to doctors.

More worringly, doctors have discovered a new strain of hospital superbug, VRSA - Vancomycin resistant staphylococcus aureus - at a Scottish hospital.

Vancomycin was thought to be the last line of antibiotics able to defeat the bacteria.