The research found that of 751 third year pupils from four secondary schools who were interviewed, more than half had part-time jobs and 96 per cent of those who worked were doing so without a licence.

The study, commissioned by BBC Scotland and carried out by Paisley University, said some of the children interviewed had suffered accidents.

The principle report author, Dr Jim McKechnie, said the most common job undertaken by children was delivery work.

He said: "The work covered things like newspaper and milk delivery, and leaflet delivery from door to door.

"You also find children working in shops, catering and door-to-door sales - 'hawking' basically."

Ban on delivering milk

The main legislation covering child employment was introduced in the Children and Young Persons (Scotland) Act 1937 and various byelaws since then.

It stipulated that children under 12 cannot work, while children over 13 can work limited hours but not before 7am and not after 7pm.

But one in five of those working children started work before 7am and almost half worked after 7pm.

The types of job open are restricted under law and children are not allowed to work on building sites, sell alcohol or sell goods on the street.

However, byelaws, such as Glasgow City Council's ban on children delivering milk, have been introduced more recently.

Injured at work

Dr McKechnie said nearly a fifth of the children had been involved in some kind of accident.

He said: "There are children who reported having a broken arm, broken toes from falling down stairs and burns from their workplace.

"All of these issues would suggest the children aren't working in harmless types of jobs. The jobs carry risks and those risks have to be attended to."

Those who employ children have to obtain a work permit from their local education authority.

However, Glasgow's director of education, Ken Corsar, said very few applications have been received.

'Law is in disrepute'

He said: "We issued eight permits last year and a similar number the year before."

Mr Corsar accepted that the number of permits issued is extremely small but said efforts are being made to rectify the problem and a publicity campaign will be launched.

Kathleen Marshall, Professor of Child Law at Glasgow University, said the law on child labour "is in disrepute".

"Nobody has any respect for it because it doesn't have any bite at all."

Professor Marshall said the law should be enforced to offer children a "protected environment".

'Employers goodwill needed'

But Mr Corsar said the city council is unable to enforce the legislation and can only work through providing information.

"We ensure that all the school staff, particularly guidance staff, are aware of the law. But we have to rely on the goodwill of employers not to break the law."

The study found that the average working week was 6.1 hours and more than half of those working did so for five hours per week or less. However, around one in eight children worked more than 10 hours per week.