By Jon Craig, Chief Political Correspondent
Obese people could lose benefits worth up to £100 a week if they refuse to diet, as a review into sickness payments is ordered by 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 the Prime Minister.
"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 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 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 Government source said: "As well as the unwarranted expense, this represents an unproductive waste of human potential."
Tam Fry from the National Obesity Forum told Sky News: "Obesity is the issue which might bring down the NHS.
"We have the most appalling problem and so far the coalition government have done absolutely nothing serious about it. If this is a sign they are taking obesity serious then I think this is something they should be considering."
He added: "You've got to be very careful about how you do it because all those people who are fat because they have a metabolic and a medical syndrome attached to it, they have got to be protected.
"But for the people who just eat and eat and eat and refuse to take any kind of treatment, then I think there is a salvo that has to be fired across their bows."
However, disability campaigner Ellen Clifford was outraged by Mr Cameron's proposal, telling Sky News: "If you threaten taking away people's benefits that's not actually going to help people. It didn't work in the Victorian times, it's not going to work now.
"That isn't going to suddenly snap people out of an enduring condition. It's punitive and it's savage."
And former Labour spin doctor Alastair Campbell, who had a well-publicised battle with booze and is an Alcohol Concern ambassador, criticised the proposal as a "stupid little stunt".
He said the Prime Minister was "pathetic" and was not taking the problem of addiction seriously.
