It might be acceptable for Lucozade and Ribena to pledge to reduce the sugar in their products to help fight obesity.

But AG Barr knows its market, especially in Scotland, and is steadfastly refusing to change the famed recipe of its main brand, IrnBru.

It has agreed to reduce the calorie content across its drinks range by 5%, while sugar in the Ribena ready-to-drink line and Lucozade Energy will be cut by "up to 10%".

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However, an AG Barr spokeswoman said it had no plans to reduce the sugar content of its regular IrnBru product "at this time" because "we do not believe customers would accept this".

She added: "We have always recognised that we have a role to play in helping our consumers exercise choice in their calorie consumption. However we can only make a recipe change in one of our products if we are 100% satisfied that the product is absolutely right for our consumers.

"Regular IrnBru is a unique-tasting product with a very loyal consumer base. Any change to the sugar level in IrnBru would impact the product delivery and we do not believe that our consumers would accept this.

"We do however offer IrnBru consumers a choice of pack sizes including 250ml to help them regulate their calorie consumption. We will also continue to maintain our focus on increasing sales of sugar-free IrnBru and its overall proportion of total IrnBru sales.

"Total sales of sugar-free IrnBru has grown by 15% over the last five years, significantly ahead of regular. Regular IrnBru is less than a third of our total business volume."

The UK has some of the highest obesity rates in the developed world, with 60% of adults and one third of 10 and 11-year-olds being overweight or obese.

The Westminster Government launched its Responsibility Deal calorie reduction pledge to encourage businesses and other organisations to help consumers consume less sugar.

The J20 brand has also announced it is to launch two flavours in a new slimline can which will represent a 10% calorie reduction compared with its standard 275ml bottle, while Co-operative Food will look at controlling calories in popular product lines and Burton's Biscuits will offer more portion control packs and review its recipes.

Responsibility Deal Food Network chairwoman Dr Susan Jebb said: "I'm pleased to see the soft drinks manufacturers like GSK, AG Barr and Britvic join Coca-Cola and PepsiCo to make some very real commitments to help consumers cut down on their calories as they take control of their weight.

"I hope we will now see others, including the out of home sector, taking a careful look at how they can build on this and come to the table with new commitments to encourage their customers choose smaller portions and swap to lower calorie options."

Food and Drink Federation director general Melanie Leech said: "We commend these businesses for joining existing pledge signatories in a shared commitment to support the calorie reduction challenge issued to the nation by ministers last year. Of the 31 pledge signatories, 15 are manufacturers, demonstrating our sector's willingness to engage and deliver improved public health outcomes under the deal."

British Soft Drinks Association director general Gavin Partington said: "We welcome the commitments made by leading soft drinks producers. Our industry has been taking steps to reduce the calorie content of our drinks for many years now, and currently more than 60% of all soft drinks contain no added sugar."

The eight new drink and food manufacturers, supermarket and catering companies to have signed up to the pledge join 23 others including Mars and Tesco.