A pill that makes the body fit without the need for exercise has been hailed by scientists following astonishing test results.

The drug fools the muscles into thinking they have worked out long and hard, which means they rapidly burn fat and boost fitness.

It has been hailed as the couch potato's dream, giving the benefits of exercise without leaving the sofa.

Scientists found the drug boosts fat burning and stamina in inactive mice

Experts say it could combat ills from obesity to muscle-wasting diseases. Even the frailty of old age could be eased by the drug's muscle-building powers, the U.S. scientists believe.

Researcher Ronald Evans, a world-leading biologist, said: 'We have exercise in a pill.'

But its arrival on pharmacy shelves is believed to be several years away.

Professor Evans, of the Salk Institute in California, showed that the drug, known as AICAR, boosts fat burning processes and stamina in inactive mice.

'Couch-potato' animals that took AICAR for four weeks burned more calories and had less fat than untreated mice.

When tested on a treadmill, they could run almost 50 per cent longer than untreated mice. Effectively, they had become fit without moving a muscle, which Professor Evans said left him 'blown away'.

'It is tricking the muscle into "believing" it's been exercised daily,' he said. 'It proves you can have a pharmacological equivalent to exercise.'

A second drug, GW1516, produced even better results, but only in conjunction with regular exercise, the journal Cell reports.

After a month of taking that drug and exercising, mice could run 70 per cent further than other mice that had worked out on the treadmill but did not get

the drug. 'The dramatic effect of the drug was stunning,' said the professor.

It is thought the drugs reprogramme the muscles, boosting the number of fat-burning fibres and allowing them to contract repeatedly without tiring.

They flick a 'master switch' in muscle metabolism, increasing the production of 'slow-twitch' muscle fibres which burn fat rather than sugar for energy.

AICAR also raises levels of an enzyme that tells muscles they are running low in energy and need to burn some fat.

The researchers say their work offers hope for the obese, those with muscle-wasting diseases and those who cannot exercise because of a medical condition.

The pills could also offer the benefits of exercise to those who cannot find time to fit the gym into their hectic working day.

'Almost no one gets the recommended 40 minutes to an hour per day of exercise,' said Professor Evans. 'For this group of people, if there was a way to mimic exercise, it would make the quality of the exercise that they do more efficient.'

Fellow researcher Dr Vihang Narkar cautioned that a pill would not be a panacea, saying a healthy diet and exercise are 'cardinal' to good health.

AICAR, the version that worked without exercise, is already in advanced human testing to see if it can prevent a particular complication of heart bypass surgery.

The second drug was being tested as a cholesterol-lowering pill. However, trials were halted over concerns about safety.

Professor Stephen Bloom, of Imperial College London, said: 'Even if these particular agents turn out to have side-effects, the proof of principle is still there and we can look forward to someone making something which is safe and effective.'

Dr Colin Waine, of the National Obesity Forum, called the research 'important' but added that at the moment the best way to tackle weight problems was with healthy eating and increased physical activity.