Some 78 per cent said that schools should not be allowed to provide the machines, which are estimated to operate in 95 per cent of secondaries and can earn schools up to pound;15,000 a year.

The survey of 736 parents by FDS International, carried out as The TES launched its Get Active campaign which aims to improve health among the young, is the third in the past six months to find a clear majority of parents opposing the machines. Last October, a poll of 1,000 people for the Guardian put the figure at nearly seven out of 10. In November, 81 per cent of parents surveyed by YouGov called for a ban.

Joe Harvey, of the Health Education Trust, said ministers should require schools to produce food and nutrition policies. After writing these plans, headteachers and governing bodies would find it very hard to justify operating vending machines which sold junk food, he said.

The Department for Education and Skills has no plans to ban vending machines. A spokeswoman said that the decision whether to install machines in schools rested with the headteacher and governing body.

She said: “We believe that they are best placed to understand the individual needs of their school and pupils.”

This month, the Government published a Healthy Eating Blueprint showing heads how they can contribute to pupils’ well-being, and highlighting a pilot project which tested using the machines to sell healthier products.

Coca Cola has also been persuaded to remove its logos from the 4,000 machines it operates in British schools. However, ministers have so far fought shy of an outright ban. Headteachers’ leaders argue that the machines generate valuable extra cash for schools, and that if they were banned, pupils would just buy fizzy drinks, crisps or sweets from shops outside school.

Ministers, who have been encouraging schools to provide drinking fountains, will be pleased by the fact that 84 of parents polled believed their school did make fresh water available.

The survey found that children spend an average of 5.7 hours each weekend watching television, 3.8 hours with friends and 3.1 hours on the computer.

In comparison, parents said youngsters spent four hours each weekend on physical activity on average - 1.6 hours on sport, 0.3 hours on dancing, 0.3 on aerobics or the gym and 1.8 on “other physical activities”.

As children get older, they appear to be spending more time indoors. Among 11 to 16-year-olds the average time spent either watching TV or on the computer rose to 9.9 hours, with only 3.8 hours spent on physical activity.

Professor Neil Armstong, director of the Children’s Health and Exercise and Research Centre at Exeter university (see panel, right), said parents may be overestimating the amount of exercise their children were taking.

The pollsters used a scoring system to analyse the nutritional value of children’s diets. Parents were asked how frequently their children consumed healthy options (fresh fruit, green vegetables and salad) and less healthy food (crisps, fizzy sugar drinks, sweets, chocolate, ice cream, chips and burgers).

The researchers then allocated one point for each healthy item consumed in a week, and - 1 point for each unhealthy item. Children with scores above zero were said to have a healthy diet, those scoring 0 to - 5, “neutral” eating habits, and those below - 5, unhealthy diets. Overall, 45 per cent were said to have unhealthy diets.