Allowing children under 15 to do paper rounds before school is "in principle contrary" to human rights law, says a European watchdog.
The European Committee on Social Rights (ECSR) says the hours UK children can work are "excessive".
Some local by-laws allow UK children as young as 13 to do "light work" like delivering newspapers or shop work.
But work considered light "ceases to be so if it is performed for an excessive duration", says the report.
UK law allows children to work up to 12 hours a week during term time, with a maximum of two hours on school days and Sundays.
It stipulates children should not work more than an hour before school and should not start work before 7am or finish after 7pm - but local by-laws can sometimes be less restrictive, the report found.
The Committee monitors how individual states apply the legally binding European Social Charter.
It is part of the Council of Europe which was set up by statesmen including Sir Winston Churchill to promote human rights in Europe and is completely separate from the European Union.
The Charter requires that during the school term, the time children may work must be limited "so as not to interfere with their attendance, receptiveness and homework", the report points out.
For example "allowing children aged 15 years, still subject to compulsory education, to deliver newspapers from 6am for up to two hours per day, five days per week before school is not in conformity with the charter", it adds.
The report also expresses concern about work by children during the school holidays.
Excessive working hours for children can risk "their health, moral welfare or education", it suggests.
In the UK, 15- to 16-year-olds can work a maximum of 35 hours a week in the school holidays, with a maximum of eight hours on weekdays and Saturdays and two hours on Sundays.
For 13- to 14-year olds, school holiday working times are restricted to 25 hours a week, with a maximum of five hours on weekdays and Saturdays and two hours on Sundays.
The Charter requires children in full-time education to have a two-week break from any work during the school holidays in a calendar year.
In the report, the committee asks the UK for confirmation that children have two consecutive weeks free from work during the summer holiday in the United Kingdom.
It asks the UK for more detailed information on how it monitors possible illegal employment of children and what sanctions are imposed against employers who do not comply with restrictions under the law.
The Department for Education did not directly address the ECSR's suggestion that the hours UK children are allowed to work are "excessive" but said there were "clear rules to protect school age children".
"They limit time that teenagers can spend working - especially within the school term," said a spokesman.