Adding water to the pill makes it swell into a large jelly-like ball

They liken the effect to eating a bowl of spaghetti and say the pill can stop hunger for a few hours.

It is made from a hydrogel, which the team developed when trying to make more absorbent nappy linings, and may help in the battle against obesity.

So far it has been tested on 20 people but experts warned bigger trials would be needed to test safety.

Professor Luigi Ambrosio, lead researcher on the study at Italy's National Research Council, realised when they developed the hydrogel it may have a similar effect to gastric banding - a surgical procedure that reduces the size of the stomach.

One of our researchers tried the pill - he took it at about 11am and was still full at six in the evening

Professor Luigi Ambrosio, study leader

The tiny pill is powdery when dry but when swallowed with a glass of water turns to a jelly-like ball in the stomach.

It is made from an organic compound called cellulose and can be flushed out by the body.

The pill, which has yet to be named, is being tested in a further 90 overweight volunteers who will be monitored to see how much weight they lose and if there are any adverse effects.

Small portions

Professor Ambrosio said the pill should be taken about 30 minutes to one hour before each meal.

It passes through the digestive system within five to six hours.

"One of our researchers tried the pill - he took it at about 11am and was still full at six in the evening.

"It's like eating a bowl of spaghetti."

He said people could still eat a meal but would only need small portions.

"We expect the results of the trial in October and they we will try to launch the product in May 2008."

Professor Paul Hatton, leader of the Biomaterials Research Group at the University of Sheffield said the idea was "intuitive".

"There are plenty of ways of making hydrogels that are safe, they are used in medicines and prostheses.

"In principle it seems possible but there would have to be a lot of safety testing."

Dr David Haslam, clinical director of the National Obesity Forum said there had been products around for many years which had tried to produce a feeling of fullness.

"The theory has been tried and tested in the past but it's not been that successful.

"We'd need much bigger trials but it's a reasonable enough idea.

"However, it would have to be given with the appropriate nutritional advice, you can't live on such tablets."