Some hospital cleaning products may actually help a diarrhoea-causing bug survive, scientists will be told today.

Elderly patients - who are most at risk from Clostridium difficile, which is the major cause of hospital-acquired diarrhoea - could have their health endangered, experts will warn.

Researchers from Leeds General Infirmary and the University of Leeds found that some strains of the bug became more resistant when exposed to two cleaning agents used in hospitals.

Using five cleaning products, they tested samples of the bug and found that those causing hospital outbreaks produced far more spores than the less dangerous strains. All the strains produced more spores when exposed to two chemicals that did not contain bleach.

Professor Mark Wilcox, who will present the findings at the Society for General Microbiology's annual meeting today, declined to name the two cleaning products. The other three products contained bleach which "gave them an advantage", he added.

Professor Wilcox said: "We have shown that some commonly used hospital cleaning and disinfectant agents not only fail to kill bacteria, they actually promote spore formation.

"The choice of cleaning agent may have a substantial effect on the persistence of Clostridium difficile..."

Clostridium difficile is not as deadly as the superbug MRSA but it has led to several deaths. Careful use of antibiotics and being able to isolate infected patients are two of the things that help to stop it spreading.

In 2004, Department of Health figures showed there were 44,488 cases of the bug in those aged over 65.