Long-term contracts with private companies are preventing schools getting rid of junk food, according to the Guardian.

The paper reports that new schools locked into 25-year contracts through private finance initiatives are finding that they cannot rid their menus of junk food, despite the government's pledge to improve the standard of school dinners.

Other schools face substantial financial penalties if they try to opt out of long-running contracts with private catering companies.

However a study by market research company ACNielsen found that school meals are already becoming healthier with a sharp decline in canteens ordering chips, chocolate and frozen turkey.