Is the person next to you washing with soap?

People are more likely to wash their hands properly after using the toilet if they are shamed into it or think they are being watched, scientists said on Thursday.

Handwashing is the cheapest way of controlling disease but less than one third of men and two thirds of women wash their hands with soap after going to the toilet, a British study by the London School of Hygiene and Tropical Medicine showed.

But when prompted by an electronic message flashing up on a board asking: Is the person next to you washing with soap?, around 12 percent more men and 11 percent more women used soap.

Health authorities around the world are stepping up efforts to persuade people to be more hygienic and wash their hands properly to help slow the spread of H1N1 swine flu, which was declared a pandemic by the World Health Organization in June.

Handwashing with soap has been ranked the most cost-effective intervention for the worldwide control of disease, the study's authors wrote.

It could save more than a million lives a year from diarrhoeal diseases, and prevent respiratory infections -- the biggest causes of child mortality in developing countries.

In developed nations, handwashing can help prevent the spread of viral infections like flu and sickness and diarrhea bugs like norovirus and rotavirus, as well as hospital-acquired infections like MRSA and C-difficile, the authors said.

The researchers studied the behavior of a quarter of a million people using toilets at motorway service stations in Britain over 32 days. Use of soap was monitored by sensors.

The study, published in the American Journal of Public Health to mark Global Handwashing Day, showed that with no reminders, 32 percent of men and 64 percent of women used soap.

The researchers then flashed a series of electronic messages, ranging from Water doesn't kill germs, soap does to Don't be a dirty soap dodger, onto screens at the entrance of the toilets and measured how behavior changed.

The message that produced the strongest positive response was: Is the person next to you washing with soap? -- showing that people respond more when they think others were watching.

The researchers also noted intriguing differences in the behavior of men and women: While women responded to simple reminders, men tended to react best to messages that invoked disgust, such as Don't take the loo with you -- wash with soap, or Soap it off or eat it later.