Nearly a quarter of children under the legal age limit of 13 have jobs and more than a quarter say they are sometimes or often too tired to do their school work as a consequence.
The survey, conducted by Mori for the TUC, found that one in five 11- year-olds and more than one in four 12-year-olds had performed paid work.
The most popular job during term and in the summer holidays for all 11- to 16-year-olds was babysitting - undertaken by around 40 per cent of schoolchildren who work. Paper rounds were taken on by more than a quarter of working children. Girls were significantly more likely to perform babysitting and boys paper rounds, partly accounting for higher injury rates among boys.
Cleaning and shop work also featured prominently in the list of tasks frequently undertaken. Some two per cent did work in factories during term time and three per cent during the summer holidays - which is illegal.
One in six youngsters between the ages of 11 and 16 works seven days a week and one in five had been involved in an accident or had been injured at work, Mori found in the poll of 4,295 pupils in 175 schools.
Nearly 40 per cent of 11- to 16-year-olds had worked before 7am or after 7pm, which is also illegal. Almost a quarter of 11-and 12-year-olds had breached the hours limit.
The TUC believes it is the first time the true extent of illegal working among children has been revealed and attacks the Government for allegedly delaying European law which would reinforce and simplify existing British legislation. The European directive on the protection of young people at work was due to be implemented in this country by last June.
While some bad employers knowingly exploit children as cheap labour, says the TUC, most are ignorant of the legal constraints and so are parents.
British law is a "confusing mix" of hundreds of by-laws, the application of which is undermined by inadequate resources, and national legislation.
John Monks, general secretary of the TUC, said that while earning extra pocket money enabled children to learn independence, their education and health should not be allowed to suffer. He said: "Many employers are probably just as confused as most parents about what the law says on employing school kids. What we need urgently are clear, standardised laws and proper resources for local authorities to enforce them effectively."
The survey found regional variations. More Welsh children worked as cleaners (23 per cent) than any other region and in East Anglia more than a quarter were employed in catering and one in 10 on farms.
The TUC commented that school-age workers received "lousy pay". Nearly four in 10 earned less than pounds 10 a week during term time and just one in 10 earned pounds 30 a week. Holiday jobs were better paid, probably because of the longer hours. During the summer, two-thirds earned up to pounds 30 a week and one in five between pounds 30 and pounds 100.
This working life
Reporters from Children's Express, a charity which provides a news service for youngsters, spent a day applying for jobs. Here is what happened to them.
Majida Khatun, 14, was offered pounds 1 per hour to stack shelves and help shoppers at a convenience store in Peckham, south-east London. "The boss told me that people with experience get more," she said.
Later, Majida was offered pounds 30 to work a 40-hour week at a hairdresser's, also in Peckham, with the promise of a further pounds 10 if she agreed to work on Saturdays. "She didn't ask me if I had any qualifications. I would have had to shampoo customers and clean up."
Fifteen-year-old Marisa Aziz (not her real name) took on a summer job at a dentist's surgery in Shepherd's Bush, west London, in June. As well as general reception work and making tea, Marisa was surprised to be asked to help with mouth suction, sterilisation of equipment and other tasks normally reserved for a trained dental nurse.
"I worked from 9.30 to 6, and I was paid only pounds 10," she said. "I told them I was 15 and they said it didn't matter ... When my parents found out, they made me quit."
Dan Redre, 15, (name also changed) said he worked for 60 hours in one week at a restaurant in north London in July. He received pounds 70. "I was a waiter. I didn't enjoy it at all," he said. "I started early in the morning and finished late at night. I didn't know how much my wages were because I was a beginner, but at the end of the week, I left."Reuse content