Secondary schools across Britain could be forced to pay more to school dinner firms or risk losing their canteen operators, because bans on fizzy drinks, sweets and salty snacks in vending machines and tuck shops have hit catering profits.

Only a few weeks into the school year, some of Britain's biggest contract caterers are already complaining that pupils, unable to get what they want from school vending machines, are buying their favourite snacks and drinks at newsagents and chip shops beyond the school gates.

Catering executives have told the Guardian that unless they receive adequate compensation for abandoning junk food, they will quit schools where they cannot make a decent return.

The development is bad news for the straight-talking spearhead of Britain's school dinner revolution, the celebrity chef Jamie Oliver, whose latest television series about the woeful standard of food in Britain's schools returns to the screen tonight.

Until now Oliver's campaign, which elicited a nod from Downing Street and approval from parents across the land, appeared to be set to transform school diets.

Earlier this month, the education secretary, Alan Johnson, announced that an original healthy ingredient subsidy, granted to schools in the wake of Oliver's first series, had been extended to 2011 with an additional investment of £240m. The sale of junk food has already been banned from lunchtime cafeterias in schools under new food regulations, and Mr Johnson made clear he expects schools to banish unhealthy food from tuck shops and vending machines used at other times during the day - a rule that will be compulsory from September next year.

Recently broadcast trailers for Jamie Oliver's new series, in which he will revisit school canteens to track their progress, have featured the chef disguised beneath several layers of fat, with chunks of hamburger stuck to his cheeks.

Channel 4 executives may now feel that the advertising ploy is an irony too far. In tonight's show, Nora Sands, the dinner lady who was featured in the original series, claims to be losing £70 a day from her budget because of the school ban on tuck, despite subsidies. Oliver tells her: "Basically, it is almost essential - for your business model to work - to sell crap."

"Yes," she replies. "Live in the real world, Mr Oliver."

Many contract caterers appear to agree with the dinner lady from Greenwich, and have stopped bidding for new contracts at schools where they cannot sell the junk food they claim children want.

Ian El-Mokadem, the UK boss of Compass, the firm best known for serving up Turkey Twizzlers, said: "As you start to reduce things that kids want to eat all day, they will just walk out and buy them elsewhere." The company's school dinners division, Scholarest, which serves one in 10 of UK school meals, is suffering from children leaving the school gates at lunchtime, he said.

"As new standards apply, there is a need for us to go and renegotiate on a school by school basis ... We won't sign up to operate in an environment where we don't think we can do a good job - provide proper quality food and a fair return for our shareholders. We won't do it."

He added that raised standards have effectively changed the terms of contracts. "[In many cases] the original brief for wider choice and a lot more vending is all gone ... There will probably be a few schools where we say: 'We can't work here any more. We can't make it work'."

Scholarest's largest rival, Sodexho, has produced its own "roadmap document" for headteachers and is encouraging schools to take a more gradual approach to the introduction of healthy snacks, resisting pressure from ministers for rapid action.

"Some secondaries have implemented early and seen great losses," a spokeswoman said. "We aim to look at what the loss could be and how we can mitigate it ... As a last resort, if we can't make money, we would have to renegotiate."

School catering companies have come to rely on vending machines and tuck shops as a rich source of profits. Some are pushing for the big confectionary groups to put more promotional money behind new vending machine products, such as muesli bars and smoothie drinks.

They hope these products will both meet the government's guidelines and appeal to children.

"Education is certainly one of the most economically challenging areas for us this year," Mr El-Mokadem added. "But we are certainly not going to cut and run."

Meanwhile, schools across Britain are also starting to feel the pinch, amidst a backlash against the arrival of fresh vegetables and fruit. One catering industry insider told the Guardian there is evidence that a number of schools are striking deals with mobile tuckshop operators, encouraging them to pitch up outside the school gates during breaks in the day.

At Rawmarsh comprehensive in South Yorkshire, where children are not allowed out at lunchtime, some parents are apparently so unimpressed with the new menus that they are passing contraband meals to pupils through the school railings. Every lunchtime a group of mothers are reportedly taking orders for more than 50 unhealthy meals, including burgers and fish and chips, from a graveyard next to the school.

The new menu

Jamie Oliver threw a spotlight on the stomach-turning standards of many school dinner caterers two years ago in a Channel 4 series that shocked parents and led to 300,000 signatures on a petition to Downing Street.

It was in part blamed for a 2% decline in school meal takeup last year, making it harder for kitchens to cover overheads. The government responded with a £220m healthy ingredient subsidy, which was this month extended to 2011 with a further £240m made available.

New standards include no sweets, soft drink, chocolate or salty snacks at lunchtime; no salt cellars on tables; and deep-fried food no more than twice a week. Dinners must include vegetables and fruit. Bread and water must be available at every sitting, and oily fish must be on the menu at least once every three weeks.