Calorie labels on fast food menus will do nothing to improve diets, a new study shows.

Just eight per cent of people will make healthy choices as a result of the scheme, scientists claim.

It will come as a devastating - yet unsurprising - blow to public health policymakers who have been trying and failing to change attitudes towards processed meats and soda for years.

The study comes just six months before a policy goes ahead that will require food labeling across outlets in the US.

Just eight per cent of people will be deterred by calorie counts on fast food menus

Some outlets in the UK, including McDonald's and Wetherspoons, have already introduced calorie counts to their menus.

Lead researcher Andrew Breck, from New York University, said: 'Health policies would benefit from greater attention to what is known about effective messaging and behavior change.

'The success of fast food menu labeling depends on multiple conditions being met, not just the availability of calorie information.'

In 2006, New York became the first city to introduce labelling requirements for to motivate fast food chain customers to change their behavior by providing them with health information.

Philadelphia and Seattle followed shortly after.

From next May, the Food and Drug Administration is requiring all restaurants with at least 20 locations to post calorie information.

But most studies in places that have already adopted the technique, including New York, have found little evidence it works.

Previous research identified five factors that need to be present including consumers being aware of the initiative and having the motivation to change.

They must also be able to estimate their daily calorie intake, be surprised by calorie counts and eat fast food at least once a week.

In the new research, they collected data from Philadelphia shortly after labeling went into effect in 2008.

They analysed responses from 699 consumers who completed point-of-purchase surveys

Mr Breck and colleagues used this framework to better understand why menu calorie labeling policies have had a limited impact.

Using data collected in Philadelphia shortly after labeling went into effect in 2008, they analysed responses from 699 consumers who completed point-of-purchase surveys at 15 fast-food restaurants, as well as responses from 702 phone surveys of the city's residents.

Based on the two surveys, the researchers found a small minority of fast-food consumers met all conditions, and therefore would be expected to change their eating behavior as a result.

Only eight per cent of those surveyed in fast-food restaurants and 16 percent of those surveyed by phone met all five conditions.

A third of those surveyed by phone did not see calorie labels posted and nearly two thirds surveyed at point-of-purchase did not notice the calorie information.

As a result, the researchers recommend restaurants make calorie information more visible to consumers through clear signage and fonts that are large and in a noticeable color.

In addition, the researchers cited past experiments showing people responded to calorie labeling on menus that included the average recommended daily calorie intake, or explained how much exercise would be needed to burn off different foods.

Three-quarters of those surveyed by phone correctly estimated the number of calories they should consume daily, but this was true of less than half of those surveyed at point-of-purchase.

The researchers also note that the visibility of calorie labelling may spur restaurants to reduce the calorie content of existing menu items and provide additional lower calorie options.

Added study author Prof Beth Weitzman: 'We know few regular fast food eaters chose fast food because it is nutritious. They instead are motivated by cost and convenience

'However, requiring restaurants to make the calorie content of their menu items highly visible could cause restaurants to add new, healthy options to their menus.'