Fans of some of the UK’s most iconic chocolate brands could soon be chomping on bars that are more than 10% smaller than they are at the moment.

As part of its Responsibility Deal Pledge, Mars promised to limit products to a maximum of 250 calories a portion by the end of 2013. But this week, its UK MD Fiona Dawson said simple reformulation might not be enough to hit the target and that some bars would have to shrink.

The company is developing a string of smaller bars, with a view to launching them next year. These include the classic Mars Bar, which currently packs 260 calories per 58g bar, and Snickers, at 280 calories per 59g bar. Other standard-sized products in the portfolio that currently exceed the limit include the 58g Milky Way, at 270 calories.

“We’re looking at the options and experimenting with a number of different sizes with customer groups,” said Dawson.

Although Mars had cut 15% of the sat fat content from Mars Bars in 2010, there was “only so much reformulation you can do”, she added.

Dawson, who is part of the Responsibility Deal inner circle that created the recent pledge to cut five billion calories from the nation’s diet, promised to tackle the issue “head on”.

“Actions will always speak louder than words,” she said. “If you look at these plans for calorie reduction and the reductions in salt, we’ve already made, these are all positive steps and I hope this latest set of announcements will answer some of the criticisms. But sometimes I think being under all this scrutiny from the health lobby is a good thing.”

The government this week confirmed three major new signatories to the calorie reduction pledge - Aldi , United Biscuits and JD Wetherspoon.