Commuters in the north were dirtier than in the south
More than one in four commuters has bacteria from faeces on their hands, an investigation suggests.
Scientists from the London School of Hygiene and Tropical Medicine swabbed 409 people at bus and train stations in five major cities in England and Wales.
The further north they went, the more often they found commuters with faecal bacteria on their hands - men in Newcastle were the worst offenders.
Experts stressed the importance of hand hygiene for preventing illness.
The bacteria found suggested people were not washing their hands properly after using the toilet, said the researchers.
In Newcastle and Liverpool, men were more likely than women to show contamination - 53% of men compared with 30% of women in Newcastle and 36% of men compared with 31% of women in Liverpool.
In the other three cities - London, Cardiff and Birmingham - the women's hands were dirtier.
People who had used the bus had higher rates of hand contamination than those who had used the train.
Manual workers had cleaner hands than other professionals, students, retired people or the unemployed.
Dr Val Curtis, director of the Hygiene Centre at the London School of Hygiene and Tropical Medicine, said: "We were flabbergasted by the finding that so many people had faecal bugs on their hands.
"The figures were far higher than we had anticipated, and suggest that there is a real problem with people washing their hands in the UK.
Newcastle - men 53%, women 30%
Liverpool - men 36%, women 31%
Birmingham - men 21%, women 26%
Cardiff - men 15%, women 29%
Euston (London) - men 6%, women 21%
"If any of these people had been suffering from a diarrhoeal disease, the potential for it to be passed around would be greatly increased by their failure to wash their hands after going to the toilet."
Professor Mike Catchpole, director of the Health Protection Agency's Centre for Infections, said: "These results are startling and should be enough to make anyone reach for the soap.
"It is well known that hand washing is one of the most important ways of controlling the spread of infections, especially those that cause diarrhoea and vomiting, colds and flu.
"People should always wash their hands after using the toilet, before eating or handling food, and after handling animals. And remember to cover all cuts and scratches with a waterproof dressing."
The HPA's monitoring of infections over recent weeks suggests that cases of norovirus - the winter vomiting bug - are rising and that the annual norovirus season is likely to have begun.
Norovirus is the most common cause of gastrointestinal disease in the UK with peak activity in terms of numbers of cases and outbreaks during the winter months, from October to March.
It has been estimated that between 600,000 and a million people in the UK are affected each year.
Professor Catchpole said: "Norovirus is highly infectious and easily spread in settings where people are in close contact with one another so good hygiene, including frequent handwashing, is really important."
The study was part of the world's first Global Hand-washing Day, dedicated to raising awareness about the importance hand hygiene plays in public health.