Chocolate bars in the UK are likely to shrink by a fifth in the next three years after health authorities told sweet manufacturers they could meet obesity targets by cutting product size.

Public Health England published new guidelines which call on food manufacturers to reduce the sugar content of cereals, breakfast goods, yoghurts, biscuits, ice creams, sweet spreads and confectionery by 20% by 2020.

The cuts are intended to reduce the number of obese children by the end of the decade and take 200,000 tons of sugar out of the British diet each year.

But the new guidance states that companies who do not want to reformulate ingredients can meet targets by simply cutting the size of their products.

PHE said chocolate firms may find reformulation difficult and that reducing size was probably the most sensible solution.

However, anti-obesity campaigners warned that without reformulation shoppers may be encouraged to buy two smaller chocolate bars.

Tam Fry, of the National Obesity Forum, said: "Simply downsizing sugary food is not the answer.

"Of the four options given to the food industry to remove 20% sugar from their products, downsizing runs the real risk that people will buy two of whatever the product is and eat substantially more.

"The preferred option is to reformulate the product and replace sugar with other ingredients."

Although other experts praised the sugar reduction targets, they questioned whether manufacturers would actually meet the voluntary commitment if there were no sanctions for failing to comply.