Chocolate bars, sweets and biscuits could shrink by as much as 20% under new guidelines designed to help manufacturers cut the amount of sugar in childrens products.

The guidelines, published by Public Health England (PHE), could see 200,000 tonnes of sugar removed from the UK market per year by 2020, in a bid to tackle rising childhood obesity levels.

The nine food categories in the programme include confectionery (chocolate and sweets), cakes, biscuits and sweet spreads, which are sub-categorised into chocolate spread, peanut butter, dessert toppings and sauces, as well as fruit spreads.

One of the main commitments in the governments Childhood Obesity Plan was to reduce the amount of sugar contained in food. The challenge, it says, is to reduce sugar by 5% by August 2017 and overall by 20% by 2020.

The 20% target has been set based on 2015 average sugar levels across the nine categories, known as the sales weighted average. The sales weighted average, PHE says, is calculated by weighting the contribution of individual products by their volume sales – a high-selling, high-sugar product will increase the sales weighted average and a high-selling, low-sugar product will reduce the sales weighted average.

According to PHE, this sets out the clear goal for a sales weighted average for sugar per 100g for each category to be achieved by 2020 and provides a figure against which progress can be monitored.

The three approaches the food industry can take to reduce sugar, PHE notes, are reformulating products to lower the levels of sugar present, reducing the portion size, and/or the number of calories in single-serve products, as well as shifting consumer purchasing towards lower or no added sugar products.

PHE will judge the success of the sugar reduction programme by measuring the net amount of sugar removed from key food categories. The principles, it says, are to encourage the industry to go "further and faster" in sugar reduction in order to improve health outcomes, but also to give it flexibility in how it meets the governments challenge.

"Many companies have already taken impressive steps to rise to this challenge but its important that everyone steps up," explains Public Health Minister Nicola Blackwood. "We should seize this unique opportunity to be global leaders in food innovation."

The announcement has been welcomed by the wider food industry. Ian Wright CBE, director general of the Food and Drink Federation (FDF), says, "Obesity levels in the UK are unacceptably high. Physical inactivity is a factor, but for many the problem overwhelmingly is with excess calories in the diet. With many of these calories coming from sugars, we support the governments highly ambitious sugars reduction drive.

"Todays report represents a constructive platform on which to build a world-leading programme of voluntary sugars reduction, right across food and drink. All parts of the food industry – manufacturers, retailers, takeaways, restaurants and cafés – need to step up. The guidelines are very stretching but manufacturers, for our part, are willing to take on the challenge."

Companies are working to overcome technical challenges and adjust food products by developing new low sugar alternatives. In some foods, portion size reductions will be necessary, he notes.

"This programme is only one piece of a much wider jigsaw of work that needs to be done to move towards better overall diets and more active lifestyles," Wright adds.

Graham MacGregor, Professor of Cardiovascular Medicine at Queen Mary University of London and Chairman of Action on Sugar, says, 

"We congratulate PHEs tremendous achievement on setting coherent and achievable sugar reduction targets in such a short space of time.

"However, the missing factor in this report is how these targets will be enforced. Weve seen over recent weeks that some companies within the food and drink industry have made great progress whilst others are seriously lagging behind and others claiming wrongly that they cant do it. Doing nothing is no longer an option – we need transparency from them about how they are meeting the targets (with clear nutritional information made available for restaurants, catering companies and other out-of-home eateries). If these recalcitrant companies dont comply we need Theresa May to bring in tough measures to ensure compliance and put public health first before the profits of the food industry."

Meanwhile, Leatherhead Food Research says the guidelines "create complex challenges that call for science-led innovation".

"Reformulation of food products is a common response to health-related issues. However, in the case of sugar reduction, this is not straightforward. Depending on the properties of the product in question, sugar can contribute much more than taste. It has preservation qualities, so plays a role in the shelf-life of a product. It also impacts texture, aeration, fermentation (for products containing yeast), bulk and visual appeal.

"Food manufacturers need to consider interactions between ingredients in a recipe to understand how sugar reduction or replacement will affect the finished product."

According to Leatherhead Food Researchs head of microscopy Professor Kathy Groves, ascientific approach known as blueprinting is an effective way to address these complex challenges.

"Blueprinting creates a technical map of a product," Groves explains. "It considers both the sensory and scientific attributes that explain its profile, drawing on microscopy, microbiology and rheology. This enables objective analysis of properties such as crunch, creaminess, lightness or smoothness. Understanding the science that underpins these attributes facilitates more intelligent and efficient product development, with reduced risk."