An appetite-suppressing chewing gum or injection could be used to tackle Britain's obesity epidemic. Scientists are developing a way to emulate the body's natural signals for feeling full using a drug based on a natural gut hormone produced after every meal.

It is likely to be developed as as an injectable drug, but the scientists also believe it could eventually be taken orally and incorporated into a gum, or used in a nasal spray.

"We weren't looking at a toxic drug, which has all sorts of side effects; we were looking at the body's own way of switching off appetite after a meal," said Steve Bloom, of Imperial College, who is leading work on the new treatment, based on a hormone produced by the body called pancreatic polypeptide (PP).

In Britain, more than a fifth of adults are obese and of the remaining population half of men and a third of women are classified as overweight. In early trials, volunteers' appetites were reduced by a fifth after being injected with the experimental new drug.

Drugs to tackle obesity often have unpleasant side effects. Orlastat prevents the absorption of fat but can cause vitamin deficiencies and has to be carefully administered; Sibutramine, originally developed as an antidepressant, can drive up heart rate and blood pressure; and Rimonobant, which reverses the "munchies" associated with cannabis use, can cause nausea.

Professor Bloom said that in contrast, PP looks as if it will be free of side effects because it already circulates in the body. The body produces the hormone after every meal to ensure eating does not run out of control. There is evidence that some people have more of the hormone than others, and becoming overweight reduces the levels produced.

Prof Bloom tested the hormone in 35 overweight volunteers who were otherwise healthy. Participants were split into two groups - one was given the hormonal jabs, the other a placebo injection. They were then asked to eat as much as they liked from a buffet meal and asked questions about how hungry they felt.

Those given the treatment felt less hungry and ate less than those who received the placebo. The effect was statistically significant, reducing the amount of food eaten by 15% to 25%.

It is not the first time scientists have looked into using the body's hormones to control appetite. Several years ago, researchers pinned their hopes on leptin, a chemical also produced naturally by the body to keep a lid on appetite. Unfortunately, it did not work in trials. "Leptin is made by fat and fat people have very high levels of leptin and develop resistance - we have tested PP in fat people and it works fine," said Prof Bloom.

Prof Bloom has been awarded £2.3m by the Wellcome Trust to develop his idea, one of the first awards made in the charity's £91m Seeding Drug Discovery Initiative.

The cash will go into developing a way to get around PP's one weakness: when it gets into the blood, the hormone is immediately broken down by enzymes and quickly becomes ineffective.

One approach is to identify how the hormone is broken down in the body and then chemically modify it to resist attack. Another tactic will be to investigate a way to put the hormone into a capsule so that it can leak out slowly into the blood over a week or so.