To date such trials in Britain have been done in "closed" conditions under tight hospital control, with volunteers' excrement being collected and treated separately.

Government scientific advisers gave the all-clear for the drug company Acambis, and St Bartholomew's hospital, London, to release the GM material to the environment.

The hospital and firm intend to begin experiments to develop a GM vaccine against diarrhoea. The advisers said they were satisfied the modified bacteria shed through 50 volunteers' guts would not replicate in the sewerage system.

Another government body, the medicines control agency, still has to approve the trials but other companies are lining up similar applications.

Acambis stressed that it might be five years before a vaccine reached the market; an effective one would need at least three strains of genetically modified E coli, the bug causing diarrhoea. The prototypes knock genes out of E coli bacteria to lessen their ability to cause diarrhoea, especially those types caught abroad.

This was the first time the advisory committee on releases to the environment, best known for monitoring trials of GM crops, has had to rule on vaccine use. The committee thought it unlikely that altered bacteria might "revert to a former state" in the sewerage system, causing a health risk.

The developers of the vaccine said volunteers would "go about their normal duties", shedding live GM bacteria in their stools for up to 38 days. They would not be allowed to travel outside England and since most of the volunteers lived near London, most GM material would enter the capital's sewerage system.