Many work in the early hours before dawn, or late at night. Some are as young as seven.
A report to be published next month by a team of academics from the University of Paisley estimates that, nationally, between 1.1 million and 1.7 million pupils in the 11-15 age group now have a job. Nine out of 10 children work illegally and their average pay is pounds 1.50 an hour.
The authors, Dr Jim McKechnie, Sandra Lindsay and Sandy Hobbs, lecturers in psychology, comment: "As a society we tend to assume that child employment is a thing of the past, as depicted, for example, in the novels of Dickens."
The academics warn that working excessive hours adversely affects the children's education. They also say there is widespread confusion over the law and inadequate policing of child workers.
The researchers questioned 2,000 pupils at 22 schools in Scotland and the north of England, and found that the proportion working at the time of the survey varied between 35 per cent and 50 per cent. As many as 70 per cent had had jobs at some time. A fifth had entered paid employment at 10 or under: the youngest age reported was seven.
"These results allow us to argue that employment is not a minority experience for pre-16- year-olds," said Dr McKechnie. "Rather, it should be viewed as the norm".
Almost all jobs require a child to have a work permit issued by the local education authority. But the team says that only 10 per cent of child workers have one.
In fact, the study suggests that the great majority of children are employed illegally. Twenty per cent of those questioned from schools in North Tyneside, Cumbria, Dumfries, Galloway and Strathclyde worked more than 10 hours a week. Some started their jobs at around 4am and worked several hours before going to school. Although it is illegal for children to work before 7am, one in three of the children reported having done so.
"Our work has revealed enough to demonstrate the inadequacies of the current government policies," said Dr McKechnie. "It shows the Government to be operating on false assumptions about children employed in contemporary Britain."
Policing of child labour is the responsibility of local education authorities, the factory inspectorate and the police, but no national figures are kept on work permits issued. Indeed, the team found a number of people who did not know of the existence of work permits for children.
The report will be published shortly in the British Journal of Education and Work.Reuse content