Putting calorie labels on menus makes little difference, because diners cut back in the short-term but eat more later, research suggests.

The study by Harvard University examined nearly 50 million transactions at restaurants in the United States.

It found that when items were labelled with calorie counts, people ate an average of 60 fewer calories for each sale.

However, by the end of the study, the reduction had dropped to just 23 fewer calories - suggesting the changes were short-lived.

Overall, such changes would amount to around one pound’s weight loss over three years, the findings suggest.

The Government has proposed introducing “clear and consistent” calorie labels on menus, as part of efforts to combat Britain’s obesity epidemic.

Britain has the highest rates of obesity in Western Europe, with two in three adults overweight or obese.

Almost one in five meals is now eaten outside the home, offiicals say, with families spending at least twice as much time eating out as those who grew up in the 1970s.

But the new research suggests the measure may have a limited impact.

The study, which was published in the BMJ on Wednesday, said: "These results imply that calorie labelling alone may not be enough to make sustainable reductions in calorie intake in fast food restaurants."