Tests on shoppers’ bags revealed half contained traces of E.coli, a lethal toxin which killed 26 people in Scotland in 1996 in one of the worlds worst food poisoning outbreaks.
Scientists also found many were contaminated with salmonella.
Reusable plastic shopping bags have become increasingly popular in Britain thanks to supermarkets and other retailers giving out millions of free ones to shoppers in the last three years.
It is estimated that there are "hundreds of millions" of bags for life in use in Britain, according to sources within Wrap, the Government's anti-waste watchdog. Because the vast majority of people do not wash their bags after each shopping trip, they could be putting themselves at risk.
The tests were undertaken by the University of Arizona, whose researchers stopped a total of 84 shoppers to check the state of their bags.
The researchers warned the levels of bacteria they found were high enough to cause a wide range of serious health problems and even death.
Children may be in the greatest danger, they added, as they are particularly vulnerable to the effects of organisms such as E.coli.
Many of the bags for life are made from jute or woven polypropylene, helping to reduce the amount of so-called "virgin plastic" used in carrier bags by 40 per cent in just the last three years.
But while they are better for the environment, the new research suggests they could be harmful to health if not cleaned regularly.
Professor Charles Gerba, who led the study said: “Our findings suggest a serious threat to public health, especially from bacteria such as E.coli, which were detected in half of the bags sampled.
“Consumers are alarmingly unaware of these risks and the critical need to sanitise their bags on a weekly basis.”
A poll revealed 97 per cent of shoppers who used eco-friendly bags never washed or bleached them.
E. coli is a species of bacterium found in the intestines of animals and humans. It is passed on through faeces and can survive in the environment.
It is usually transferred to humans by ingesting contaminated water, or contaminated food, such as meat, which has not been cooked properly.
A particularly nasty strain, known as E. coli 0157, can be lethal for children and older people and fewer than 100 of the tiny organisms can cause illness.
Most cases of E. coli in Britain are caused by children coming into contact with animal faeces. Cases are on the increase according to the Health Protection Agency, which said that there were 25,532 reported cases in 2009, a 7 per cent increase compared with 2008.
Ten children last year were admitted to hospital after an outbreak at a petting farm in Surrey.
A spokesman for Wrap said: “Recently there have been concerns in the press that there are health risks from reusing shopping bags in relation to poor hygiene. WRAP recommends that only clean bags in a good state of repair are used and that bags contaminated with food should be cleaned before reusing. Bags that are in poor state of repair should be recycled at carrier bag recycling points.”