It claims the increase had put school meals - often the only hot meal that many children get in a day - further out of the reach of low-paid parents.

More than four million meals are served up every day in schools but prices vary tremendously.

In Scotland children pay as little as 65p for their lunches while pupils in the London borough of Brent pay almost twice that amount - Pounds 1.25. The average cost for a primary school meal was Pounds 1.03.

The survey discovered that five authorities did not provide comprehensive county or boroughwide school meals service.

Buckinghamshire has not offered one since 1987, except in special schools. A similar system exists in the London borough of Kingston and the service was abolished in Somerset in 1991.

The survey claimed that local authorities got round their duty to provide free school meals to pupils whose parents are on very low incomes by giving the children sandwiches. It discovered that two authorities - Dorset and the London borough of Harrow - had no hot meals service for primary children.

The majority of primary schools offered a traditional hot meal, but a number, including those in Knowsley and West Sussex, now had cafeterias. In the secondary sector the majority of schools ran a cafeteria service. The exception was Cleveland county council which had gone back to providing a set meal.

The survey revealed a growing trend among authorities for reducing or removing their subsidies to the school meals service.

Keith Sonnet, Unison’s head of local government, said: “For many children school dinners are their only hot meal of the day and Unison is very worried that commercial decisions are being made with little regard for nutritional standards.”

“School meals have an important role to play in the health of the nation and the national curriculum. School meals budgets are often targeted by local authorities to achieve cuts and our children are having to pay the price. ”