Public Health England has revealed plans to cut "excess" calories in a bid to tackle the childhood obesity crisis.
The agency says that the focus on calories is in conjunction with the soft drinks sugar tax, due to begin in April next year.
This levy will create an additional charge for drinks manufacturers whose products exceeded around 5% sugar content.
PHE said "real progress has been made" on reducing sugar levels in food items since the childhood obesity plans was introduced last year - but leading officials are calling for further action to ensure targets are met.
The comments come days after figures showed that more than 600 children and young people under 25 were treated for type 2 diabetes in 2015/16 - nearly 80% of whom were obese.
Obesity Health Alliance (OHA) said that the government "must try harder", despite the progress over the last 12 months.
It warned that the guidelines should be extended to include saturated fats, along with measures to reduce the impact of junk food marketing where children were the target audience.
OHA alliance lead Caroline Cerny said: "Good progress has been made over the last year with strong measures to reduce sugar from foods and drink and we look forward to seeing ambitious targets for industry to make the food we eat less calorific.
"However, we can't ignore the fact that the food industry continues to get away with bombarding children with adverts that we know encourage unhealthy food choices.
"The existing regulations are full of loopholes and don't reflect the way children watch TV or online content. Failing to tackle this area is significantly undermining the impact of the childhood obesity plan.
She added: "After one year, it is scraping along with a C grade, rather than topping the class with an A star."
OHA steering group member and professor of endocrinology at the University of Oxford, John Wass, also warned that the proposals did not go far enough.
He said: "2016 marked the launch of the Government's first comprehensive childhood obesity plan. But 2016 was also the year childhood obesity rose to a new, ferocious high."
"The Government pitched this obesity plan as the start of the conversation. We agree this is just the start. The stronger elements of the plan are progressing well, but these alone are not enough.
"We now urgently need to resume that conversation, to give us the best chance of protecting our children's future health."
Graham MacGregor, professor of cardiovascular medicine at Queen Mary University of London, and chairman of campaign group Action On Sugar, said: "We need a much more robust plan with enforcement of the sugar and calorie reduction targets, at the same time, the sugar-sweetened soft drinks levy needs to be extended to confectionery, the second biggest contributor of energy intakes in children.
"We must also have watertight restrictions on marketing unhealthy foods to children, with uniform front of pack labelling.
"Without the above, our children will continue to become obese and die prematurely from type 2 diabetes, heart disease, strokes and cancer."
Mr MacGregor also criticised Theresa May for having "watered down" the childhood obesity plan "for entirely political reasons" since its inception by David Cameron's government.
PHE said it will now undertake a period of detailed consultation with leading foods and drinks businesses to formulate a plan that would best enable them to reach the target 20% sugar reduction in key foods by 2020.
Chief executive of PHE Duncan Selbie said: "A third of children leave primary school overweight or obese and an excess of calories - not just excess sugar consumption - is the root cause of this.
"We will work with the food companies and retailers to tackle this as the next critical step in combating our childhood obesity problem."