The company is trialling a limited edition caramel bar, which has 204 calories compared to the 260 calories in a normal size Mars bar.
It is not known how long the trial will last for but Mars refused to rule out the possibility of making the bar a permanent addition to its portfolio depending on sales.
A full-fat version of Mars Caramel is already available in the US, Canada and Australia. The company says the caramel flavoured-sweets global market is now worth £145m.
The Snickers and M&Ms maker has promised to stop selling chocolate products with more than 250 calories as part of its commitment to the Government’s Responsibility Deal. It first announced the initiative earlier this year and has since revealed plans to make its bars more than 10% smaller than they are at the moment.
Mars has said that reformulating the recipes used to make its sweets will not be enough to meet its pledge and instead promised to invest in developing new sizes for different consumer groups. Earlier this year, the company launched an innovation hub to work on several projects, including developing lower calorie variants.
In 2007 the business said it would stop buying advertising time or space if more than a quarter of the audience as likely to be under 12 years old.