Diners will see how many calories are in each dish on cafe, restaurant and takeaway menus in England from today.
Food outlets with more than 250 employees are being ordered to display calorie counts on non-prepacked food and soft drinks on menus, takeaway websites and food labels.
The shake-up comes as part of Boris Johnson's plans to tackle obesity, after he admitted he was "too fat" when he was hospitalised with Covid in April 2020.
Mr Johnson previously opposed "sin taxes" and Government "nannying" in the battle to slim down the nation - but ditched his resistance after becoming seriously ill with the virus.
Nearly two thirds (63%) of adults in England are overweight or obese and one in three children leave primary school at an unhealthy weight.
It is estimated that overweight and obesity-related conditions cost the NHS £6.1 billion each year.
But some campaigners have warned the move to list calorie counts on menus could harm people battling eating disorders.
Tom Quinn, director of external affairs at Beat, said the eating disorder charity was "extremely disappointed".
He said: "We know from the people we support that including calories on menus can contribute to harmful eating disorder thoughts and behaviours worsening.
"For instance, it can increase a fixation on restricting calories for those with anorexia or bulimia, or increase feelings of guilt for those with binge-eating disorder.
"There is also very limited evidence that the legislation will lead to changed eating habits among the general population."
The Government's food tsar Henry Dimbleby also questioned whether the move would impact people's eating habits.
Mr Dimbleby, who co-founded the chain Leon, said: "If you look at what calorie labelling might do, I think it is a small intervention.
"The primary benefit, I think, actually, and from my experience at Leon - which we sold last year, I should say, so I'm not involved at the moment - it will make companies think a bit more carefully about what they put on the menu.
"But for the customer, I think, maybe in everyday eating out, places you go all the time, it may mean that they eat slightly lower calories but I doubt it will have much effect for eating out."
A Department of Health and Social Care spokesman said: "Obesity is one of the biggest health issues we face as a country and clear food labelling plays an important role in helping people make healthier choices for themselves and their families.
"We are all used to seeing nutritional information on products sold in supermarkets and displaying calorie information on menus can help us consume fewer calories when eating out or getting a takeaway.
"The regulations will also allow businesses to provide menus without calorie information at the request of the customer."
