From September, all school lunches in primary schools must comply with nutritional standards for limits on fat, sugar and salt content in every meal served in the canteen.

Caterers will also have to guarantee minimum levels of vitamins, proteins, minerals and fibres in their recipes.

The move will leave schools playing an increasing role in ensuring children get a balanced diet and is the final phase of the Government's programme to reform school dinners.

Joe Harvey, the director of the Health Education Trust charity in Greens Norton, which promotes healthy eating, welcomed the measure.

He said: "This is a very intelligent move, because it does analyse the nutritional value served to children. They have been created to offset any deficiencies in their diet.

"In a lot of cases, children are consuming far too much sugar and fat, but not enough calcium and zinc, which are vital for growth.

"There is a big difference between the previous basic food standards, introduced in 2006, and the new one. This nutritional-based approach means there is a better balance of food, particularly if it is not being consumed at home."

Under the guidelines, primary school meals should contain 530 calories, with a leeway of 26.5; in secondary schools, it should be 646, give or take 32.3 calories.

A spokesman for Northamptonshire County Council, which is in charge of education, said: "The local authority continues to lead the way in supporting the county's schools over radical changes to school food.

"The Healthy Food and Drink Team has been working with all 34 caterers, our central catering contractor and schools services operating in schools to ensure compliant menus for September."

Pupils' hot dinners have undergone a huge transformation since celebrity chef Jamie Oliver began his campaign to remove junk food from the canteen menus of the nation's schools.

The basic food guidelines in 2006 saw confectionery and snacks high in salt and sugar, along with fizzy drinks, banned in school canteens and vending machines and from being served at breakfast and afterschool clubs.

The new rules only apply to lunches and have been developed by the School Food Trust, a Government body. They will apply to secondary schools from 2009.