Obese people in Britain could lose benefits if they refuse to diet, under a stg£500m (€676m) crackdown on disability handouts ordered by the Prime Minister David Cameron.

Drug addicts and alcoholics could also lose benefits if they refuse treatment that would help them get a job, in a move aimed at stopping people claiming as a "lifestyle" choice.

"Too many people are stuck on sickness benefits because of issues that could be addressed but instead are not," said Mr Cameron.

"Some have drug or alcohol problems, but refuse treatment. In other cases people have problems with their weight that could be addressed, but instead a life on benefits rather than work becomes the choice."

"It is not fair to ask hardworking taxpayers to fund the benefits of people who refuse to accept the support and treatment that could help them get back to a life of work."

"The next Conservative government is determined to make sure that the hardest to help get the support they need to get them back to a fulfilling life."

Ministers estimate there are almost 100,000 people claiming sickness benefits on the grounds of treatable conditions such as drug or alcohol addiction, or obesity.

At present, there is no requirement for such people to undertake treatment, meaning it is possible to claim without making efforts at recovery.

Of the 2.5 million there claiming sickness benefits, about 1.5 million have been claiming for more than five years.

Mr Cameron says he has asked Professor Dame Carol Black to undertake a rapid review in to how best to help those suffering from long-term yet treatable conditions back in to work.

"In particular, I have asked her to consider whether people should face the threat of a reduction in benefits if they refuse to engage with a recommended treatment plan," he said.

"It is vital that people who would benefit from treatment get the medical help they need."

Professor Black, a leading UK government adviser on health, work and welfare, said: "I am deeply interested in trying to overcome the challenges these types of benefit claimants pose."

"These people, in addition to their long-term conditions and lifestyle issues, suffer the great disadvantage of not being engaged in the world of work, such an important feature of society."

Explaining the reasons for the threat to axe benefits from the obese, drug addicts and alcoholics, a British government source said: "As well as the unwarranted expense, this represents an unproductive waste of human potential."