One in four children with part-time jobs say they are sometimes too tired to do their schoolwork while one in five has reported a serious injury, according to the first national survey of child labour, published this week by the TUC.
The study, conducted by MORI, found one in four under-13s had been working for money, even though this is against the law.
According to the TUC, ever more children are working longer hours, damaging their health and education. Nearly 40 per cent of secondary school pupils have some kind of job, says the research.
John Monks, the TUC general secretary, this week demanded that the Government implement the EU Directive on the Protection of Young People, which limits all school-age workers to 12 hours a week and bars under-15s from most types of job, including shop and delivery work. The current British legislation allows 13-year-olds to work 17 hours and 15-year-olds to work 20 hours.
Mr Monks also called on local education authorities to fulfil their legal obligations and enforce the complex web of byelaws that regulate child labour.
"I'm all for schoolkids earning a bit of extra pocket money to buy CDs or clothes, learning to be independent and finding out about the world of work if they want to," said Mr Monks. "But we can't allow kids' education or health to suffer because they are working illegally and in fact doing part-time jobs. "
The Department of Health recently came under fire for proposing to relax Sunday working laws for young people, raising the total of permissible hours from two to 10 for 15-year-olds.
Although the weekend total will not change, critics say the move will encourage more children to be dragged into the Sunday trading boom and tempted to break the law and work longer hours.
The current laws are rarely enforced. In theory, child working is policed by local education authorities. In practice, very few LEAs employ full-time officers to do this. Wolverhampton and Solihull are notable exceptions.
Steve Priel, a spokesman for the GMB general union which has sponsored much of the research to date in this area, blamed other LEAs for failing to uphold the law. "Local authorities spend far more controlling car parking than on enforcing the laws on child labour," he said. "The good work done by Wolverhampton LEA puts most of the others to shame."
The GMB has been so concerned about the exploitation of child labour that it has recruited schoolchildren in Newcastle as members at a nominal rate. It is now preparing for a recruitment drive across the northern region.
Chris Pond, director of the Low Pay Unit said, "If ministers are still not satisfied of the need for action they should institute their own official investigation, as recommended by the House of Lords several years ago. They should also implement the EU directive and scrap plans for an extension of children's employment on a Sunday."
Working Classes: a report on school-age labour in England and Wales is available at Pounds 2.50 from TUC Publications, Congress House, Great Russell Street, London WC1B 3LS.