Thousands of climbers trekking up Mount Kilimanjaro are taking unnecessary risks with their health, experts warned today.

Travel firms have seen an increase in bookings following the successful summit by nine celebrities for last year's Comic Relief campaign. But researchers from the University of Edinburgh are warning that many of the people climbing Africa's tallest peak "know little or nothing" about high altitude, which can prove fatal.

Scientists tested levels of altitude sickness among more than 200 people climbing Kilimanjaro. They found that almost half (47 per cent) were suffering from altitude sickness before they reached the summit and most were ascending too high, too quickly.

Signs of sickness include vomiting, headaches, difficulty sleeping and sometimes problems with co-ordination. Effects can be felt from as low as 2,500m above sea level and 75 per cent of people will have mild symptoms at 3,000m or higher.

The best way to acclimatise is to climb slowly and some trekkers incorporate acclimatisation rest days. Some also opt for anti-sickness drugs, although their efficacy is disputed.

Stewart Jackson, who conducted the study, published in the journal High Altitude Medicine and Biology, said: "We found that many climbers knew little or nothing about altitude sickness. Undertaking an acclimatisation trek before attempting to summit Mount Kilimanjaro offers climbers the best chance of a safe, successful summit."

Severe altitude sickness can lead to serious complications, including shortness of breath at rest, inability to walk, decreasing mental alertness and a build up of fluid on the lungs which can result in a "gurgling" sound when breathing.