A loophole which exempts some academies from signing up to strict school food standards must be closed to ensure all youngsters are eating healthy dinners, town hall bosses have warned.
Around a million youngsters are currently attending schools in England that have yet to sign up to tough rules that were introduced in January last year, according to the Local Government Association (LGA).
The new standards, putting restrictions on sugary, fried and fatty foods to help ensure pupils eat a healthy diet, apply to all council-run schools as well as many academies and free schools.
But they are not mandatory for academies that opened between 2010 and 2014, although they have been asked by the Government to sign up.
The LGA said it had calculated that 2,476 schools in total have not formally committed to the standards.
As well as being able to offer fried foods and sugary drinks, it also means these schools are not required to ensure that children are offered at least one portion of vegetables or salad each day as part of their lunch, the association argued.
It added that it is essential that the Government uses a new childhood obesity strategy - due to be launched this summer - to close the loophole and force academies to commit to the food standards used by other state schools.
Official figures show that three-and-a-half million children are obese, the LGA said, putting them at a higher risk of health problems such as diabetes, heart disease and cancer.
LGA Community Wellbeing spokeswoman Izzi Seccombe, said: "It is deeply worrying that hundreds of academies and free schools are yet to commit to providing healthy school meals to children, more than a year since they were first asked to sign up to new school food standards by Government.
"It's not right that we have rules for some but not all.
"The forthcoming childhood obesity strategy is a great opportunity for the Government to close this loophole in legislation, which will make all academies follow standards that demonstrate a nutritional safety net to parents, who can be assured that their children are eating healthy food at school, rather than meals that could be laden in high amounts of fat, salt or sugar."
Plans to overhaul food standards were announced in 2014 as part of a wider shake-up of school dinners.
Ministers said that they wanted to simplify the regulations to make them easier for schools to understand.
The new rules do not set out specific nutritional content, but instead state the types of food and drink that pupils should be offered, for example fruit and vegetables on a daily basis and restricting the amount of fried and fatty food available.
A Department for Education spokeswoman said: "The new School Food Standards are ensuring more pupils eat good food that encourages healthy eating for life. They also make it easier for school cooks to devise nutritious, tasty and varied menus.
"Parents rightly expect every school to serve children a healthy lunch. All new academies are expected to meet the food standards and we are pleased that more and more academies are signing up voluntarily."