A government-commissioned food report has accused M&S of of "trickery" over the ingredients in Percy Pigs - including sugar.
The National Food Strategy, led by Leon restaurant founder Henry Dimbleby, singled out the popular treat for boasting it has "No artificial colours or artificial flavourings" and is "soft gums made with fruit juice”.
Today's report says: "These can be found near the tills at Marks & Spencer, within spontaneous reach of tiny hands.
"How many parents take the time to check the ingredients list? If they did, they might be agog to find that the three largest ingredients by weight are glucose syrup, sugar and glucose-fructose-syrup.
"I single out Marks & Spencer here, not because it is the biggest sinner, but because it is such a well trusted company.
"A British institution, M&S has the pledge 'we always strive to do the right thing' as one of its guiding principles.
"If M&S – which is a great deal more scrupulous than many food companies – is guilty of such trickery, you can be sure the practice is ubiquitous."
M&S hit back at the report, saying the boast that Percy Pigs have no artificial flavourings are true and a range with a third less sugar was introduced last year.
But Mr Dimbleby attacked Marks & Spencer over its Percy Pig sweets in a briefing to journalists.
"I just think that is not right," he told reporters. "I think that is genuinely misleading.
"And actually, when you look at the food world, the reason I pick M&S is because they have integrity as one of their values, but it is rampant in the food world - you know, low fat (food) which is actually high in sugar, or free from this and that.
"I think CEOs do have to respond to commercial pressures but they are not innocent, bobbing about on the waters of commerce, unable to take any value-based decisions.
"I think they do need to take a look at what they're doing. I think boards in these companies very quickly, if they were to ask these questions, could, without any need for regulation from Government, improve the system a lot.
"I think it's time they realised that they have been putting their head in the sand for too long."
Today's 110-page report said urgent Government action is needed in the face of the Covid-19 pandemic, and described the country's diet as a "slow-motion disaster".
It warned that poorer children risk being "left behind", adding: "One of the miserable legacies of Covid-19 is likely to be a dramatic increase in unemployment and poverty, and therefore hunger.
"The effects of hunger on young bodies (and minds) are serious and long-lasting, and exacerbate social inequalities."
The study proposes an expansion of free school meals in England to every child where a parent is receiving Universal Credit, adding: "Children who are hungry at school struggle to concentrate, perform poorly, and have worse attendance records."
At present, only children from households earning less than £7,400 before benefits are eligible.
Expanding the programme could reach an additional 1.5 million seven to 16-year-olds at a cost of £670 million a year.
The report also calls for an expansion of the holiday activity and food programme to all areas in England, reaching an extra 1.1 million children at a cost of £200 million a year.
And it urges an increase in the value of Healthy Start vouchers to £4.25 per week and expansion of the scheme to pregnant woman and households in receipt of Universal Credit with children under four.
An M&S spokesperson said: “All our products have clear labelling so that customers can make informed choices about what they buy.
"All our Percy Pigs are made with natural fruit juices and no artificial colours or flavourings and last year we also introduced a range of Percy Pigs with one third less sugar."
