A drug that has been shown to cut the risk of diabetes by two thirds has been hailed by doctors as giving new hope to thousands of people.

The world is in the grip of an epidemic of diabetes, with almost two million people affected in the UK alone and a million more predicted to succumb by 2010. The condition shortens lives by a decade and is the leading cause of blindness and amputations.

The accelerating rise in the disease is driven by obesity and inactive lifestyles, and experts have warned that the numbers could overwhelm the NHS and lead to the first reduction in life expectancy in 200 years.

An international trial of the drug rosiglitazone, run in 21 countries including the UK, has shown that it reduced the incidence of Type 2 diabetes by over 60 per cent in high-risk individuals.

But Diabetes UK warned that the results could raise false expectations and undermine efforts to get sufferers to change their lifestyles. Public health experts said the cost of the drug - £25 a month in the UK - and lack of long-term results meant governments were unlikely to pay.

Hertzel Gerstein, of the Population Health Research Institute at McMaster University, Canada, who led the study, said yesterday that the team of hundreds of researchers were "quite pleased" with the results, but warned: "No one drug is going to stop an epidemic of this size. The only way to do that is to re-engineer society so people change their diets and increase physical activity." He added that every person should understand their risk related to their family history, weight and lifestyle but that the drug could offer an "additional option".

The results of the trial, presented at a conference in Copenhagen yesterday, and published online in The Lancet, were based on over 5,200 people with impaired glucose tolerance, an early warning sign of diabetes, selected from almost 25,000. After three years, the researchers found 658 individuals on a placebo had developed diabetes, compared with 280 on rosiglitazone. There was a small increased risk of heart failure, which was not fatal, in the group taking the active drug.

The authors said: "The results of this study suggest that the addition of rosiglitazone to basic lifestyle recommendations substantially reduces the risk of developing diabetes by about two thirds, offering a novel preventative approach."

Dr Gerstein said the drug prevented one case of diabetes in every seven treated. He denied it would undermine efforts to get people to change lifestyles and lose weight if a drug could prevent the adverse effects for them.

Rosiglitazone is manufactured by the British-based multinational company GlaxoSmithKline, and is marketed in the UK as Avandia. It is used to treat diabetes, but is not licensed as a measure to prevent it.

A spokesperson for Diabetes UK said: "Research has shown that the risk of diabetes can already be cut by two thirds by reducing weight to the normal range and increasing activity. Lifestyle adjustments must go hand-in-hand with drug treatment or they will be ineffective. That is why we are backing off from this study."

A commentary published in The Lancet by two public health experts from the University of Helsinki said: "The high cost of therapy and the lack of long-term data mean that healthcare funders are unlikely to see rosiglitazone as an appropriate agent."