Didier Schmitt, head of the agency's Life Sciences and Appliances department, told journalists before the start of a space medicine conference in London on Friday that the 24 volunteers would not be able to stand or sit.

"This is the biggest experiment of this kind in Europe," he said.

It is designed to look into the effects of weightlessness that astronauts may encounter on the International Space Station or during a long manned journey to Mars.

The volunteers will lie at an angle with their feet slightly above their heads to simulate weightlessness, in which the head and chest fill with body fluids because of lack of gravity.

"The fact that they lie horizontally and are not moving is similar to the effects of microgravity," Schmitt said, adding that volunteers would start to lose bone mass very quickly.

The Americans and Russians have conducted similar experiments. The Russians hold the record with two volunteers in bed for a year, but the ESA project will be the largest with 24 volunteers.

Apart from learning about the impact of weightlessness on muscles, bones and joints and the mental and visual disorientation of spending 90 days in bed, Schmitt said scientists also hope to find out something about group dynamics.

Only men will take part in the experiment but women volunteers may be recruited for future projects.

Schmitt was one of more than a dozen scientists from ESA, NASA and Britain's National Space Biomedical Research Institute attending the conference.

"Scientists have long realized that astronauts lose bone density and muscle mass as their bodies adapt to zero-gravity conditions, but we now also know that virtually every organ in the body is affected," said Kevin Fong of University College, London, who organized the conference.