British scientists are working on a baby formula which would chemically restructure the metabolic system of children to ensure they never became obese.

Studies in mice have found that large doses of the appetite-controlling hormone leptin during infancy permanently prevent excess weight gain and reduce the chances of type 2 diabetes.

Now researchers at the University of Buckingham say a leptin-enriched baby milk which does exactly the same is less than 10 years away, raising a plethora of medical, legal and ethical questions.

Other specialists in the field condemned the search for a medical answer to obesity, saying it is a modern social ill and that people need to address their lifestyles, not look for an artificial quick fix.

More suggested the translation to baby food would be impossible as people would not put their children forward for trials of the formula when they did not know the risks involved.

The research is reported in the journal Chemistry and Industry today.

Leptin turns off appetite throughout life, but the scientists last year proved that high doses in mice through pregnancy and early life permanently reduced weight. They now believe it plays a role in hard-wiring the brain's appetite response in infancy.

Mike Cawthorne, who led the researchers, said: "The supplemented milks are simply adding back something that was originally present: breast milk contains leptin and formula feeds don't.

"Yes, it raises ethical questions. Obesity is a social problem, but it's also a health problem which costs us millions of pounds a year and is getting worse. It's not just a social problem.

"New ideas always face scepticism, but I think this is very, very likely within several years' time."

Previous experiments in treating obese people with leptin have failed as people continued to overeat. And though some research has linked bottle-fed babies to childhood obesity, none has concluded that breast-fed babies resist obesity throughout life.

Nick Finer, clinical director at the Wellcome Clinical Research Facility at Addenbrookes Hospital, Cambridge, said: "The concept that adding something to a food that could permanently alter brain development is exciting but at the same time so scary that it would mean a wholly new approach about how such treatments can be tested and approved for use. Would the first trials be in newly born children?

"The importance of leptin (and other hormones) at determining the development of brain circuits that control energy balance is an area of current research interest. The leap to a functional food being effective or safe is enormous."

Stephen O'Rahilly, head of the Diabetes, Obesity & Insulin Action department at Cambridge University, described it as "science fiction". Francesco Cappuccio, professor of cardiovascular medicine and epidemiology at Warwick University said: "Leptin is very easily destroyed by stomach acids so I'm not sure how they think they will get it ingested through baby milk."

Prof Cawthorne, whose work is being funded by a government research council and a private individual who has no connections with industry, said babies could ingest leptin because their digestive systems were less developed.

Whether or not infant formula with leptin should be classified as a food or medicine was a question that would have to be resolved, he said.

"It is still a grey area," he added. "One could argue that as you're replacing something that should be there, it's not pharmaceutical."

A spokesman for the Food Standards Agency said: "If you make a functional food using ingredients that are already on the market, then you wouldn't have to go through a safety assessment. But if it includes new ingredients then you would, and it depends on the sort of ingredients."