A staple of lunch boxes and peckish workers across the UK - Cadbury's Dairy Milk - will come in a low-sugar version from next year.
The new bar will contain 30% less sugar, a bigger reduction than the government had requested.
However, the full-sugar version will continue to be sold as usual.
Last year, Public Health England (PHE) challenged the food industry to reduce sugar levels by a fifth by 2020, or reduce the size of unhealthy products.
Mondelez, which owns Cadbury, said it was committed to tackling obesity, including childhood obesity, in the UK.
Sugar and fat
However, health campaigners said that Cadbury and other firms should go further.
"We're surprised Cadbury is offering a new lower sugar chocolate bar yet not reformulating the whole of its product range," said Kawther Hashem, nutritionist at Action on Sugar.
She said confectionary makers should be forced to make their products with lower sugar and fat content.
"For those manufacturers who don't comply, a tax of 20% (minimum) should be added to all their sweet and chocolate confectionery," Ms Hashem said.
Earlier this year, the government introduced a sugar tax on soft drinks, but some have called for high-sugar foods to be taxed as well.
The British Retail Consortium (BRC) has argued in favour of mandatory controls on unhealthy food.
Many drinks manufacturers, including AG Barr - which makes Irn Bru - and Britvic have reduced sugar levels since the tax came in in April.
Britvic says that 94% of its drinks portfolio is sugar-tax exempt. The levy starts to take effect at 5g of sugar per 100ml.
A spokesperson for Mondelez said the new bars would taste "subtly different" to the full-sugar version, but they would "maintain the same taste profile".
The company has already introduced a 250 calorie cap on all small chocolate bars, and sells a Dairy Milk bar with 98 calories.
What does it taste like?
Some BBC staff put the new lower sugar bar to the test. Were they able to tell the difference between this and the original?
Here's what they said:
- Lovely - very similar. But for full aficionados of Dairy Milk the full sticky sweetness is missing
- I was rather surprised! It tasted almost identical. Except it does leave an after taste (still tasting it now) that's not so pleasant! I'm struggling to describe what this is. It's like that after taste from fake sugar products…. If that makes sense?
- It's less sweet (unsurprisingly). In standard Dairy Milk the sugar masks the fact that it's not actually very good chocolate. And in the reduced sugar version there's nowhere to hide. And if it's not lower calorie, what is the point?
- The low sugar chocolate bar tasted a tiny bit bitter, but it was still nice.
- You could taste a lot more of the coco bean because the taste wasn't being masked by sugar.
- It tastes really totally normal!
Next year it also plans to reduce the amount of sugar in Wine Gums and Jelly Babies.
Tam Fry chairman of the National Obesity Forum said he was very encouraged by such moves.
He said that firms like Cadbury have "seen the writing on the wall" after the government introduced a sugar levy on drinks earlier this year.
"There is a real push for confectionary manufacturers to reformulate their products so that we get the quantity of sugar down as far as possible," he said.