Millions of people could lose more than 12kg by taking a pill described as a "gastric band in a tablet", a study suggests.

The results, unveiled at the world's largest obesity conference, showed the pill can be an effective alternative to weight-loss surgery.

Experts said the health services should now consider funding the pills. After the pill is swallowed, a balloon swells up in the stomach when it is filled with water, restricting the amount of calories a person can ingest.

The study of 42 adults found that they lost about 16kg on average after four months.

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The pill is licensed for use, but not yet available on the NHS in Britain. It costs around £3000 privately - around half the price of stomach stapling.

Researchers from the University of Rome said the balloon technique could be used widely and bring "significant cost savings" to health services in the long run.

The head of the NHS has recently warned that obesity is the greatest challenge facing the service, with £10 billion spent on treating diabetes.

The research, presented at the European Congress on Obesity in Porto, found that patients who were given the pill lost an average of 16kg - over 14 per cent of their total body weight.

They also saw significant improvements in their health, including blood pressure, cholesterol levels and blood sugar control.

The ease of the technique, which avoids anaesthetics and surgery, meant it could be popular among failed dieters, researchers said.

The balloon is contained inside a pill which is swallowed with water. The pill is attached to a thin tube, meaning the balloon can then be filled with almost a pint of water after it reaches the stomach. The tube then detaches and is removed. The expanded balloon fills the stomach, making the patient feel full and restricting their food intake.

Experts said counselling was necessary to ensure patients kept the weight off once the balloon naturally passes out of the body after several months.

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Dr Roberta Ienca, of the University of Rome, said: "Because the Elipse Balloon does not require endoscopy, surgery or anaesthesia, this may make it suitable for a larger population of obese patients."

Professor Jason Halford, of the European Association for the Study of Obesity, said "millions" of people could benefit from the pill if the NHS would agree to pay for it.