The bacteria that cause the problem are enterotoxigenic E. coli (ETEC). There are many forms of E. coli and most of them are harmless to humans.

However, when people travel abroad, they are exposed to unfamiliar forms of ETEC. This exposure can cause severe diarrhoea.

The new vaccine is produced from a form of ETEC that has been genetically modified so that it will not cause diarrhoea in humans.

Researchers at the Cambridge-based company Peptide Therapeutics hope that when holidaymakers drink the medicine, known as Oral ETEC Vaccine, their bodies will develop immunity to the bacteria.

They announced on Thursday that they have gained approval from American regulators to test the drug on human volunteers.

Side-effects

They have started tests on 30 people in America to see whether the vaccine is safe or whether it causes unwanted side-effects.

Unfamiliar climes bring unfamiliar bacteria

About 50% of travellers are thought to develop stomach bugs abroad. Diarrhoea can cause dehydration, and if left untreated can lead to death.

Previously, treatments were only taken after symptoms had made themselves felt.

A successful vaccine would prevent the symptoms developing at all.

The researchers hope that one dose, taken before going abroad, would keep illness at bay.

However, as part of the trials they will look into whether or not booster doses would be needed on longer trips.

Further trials

Scientists believe that five or six particular strains of ETEC are responsible for causing travellers' diarrhoea.

The trial announced on Thursday is a phase one trial, the earliest stage of testing on humans. It will look at a vaccine for one strain, and further phase one trials will look at other strains.

Eventually the researchers will produce a vaccine that guards against all the types of ETEC connected with travellers' diarrhoea.

Gordon Cameron, finance manager for Peptide Therapeutics, said it would be some time before a complete vaccine could go on sale to the public

He said the earliest it would get onto the market would be 2003.

However, the financial rewards for the company and the benefits for travellers will be huge if the vaccine proves to be successful.

It is estimated the overall vaccine market for travellers will exceed £2bn by the end of this decade.