One father attacked the policy at Sint-Pieterscollege because it threatened to punish children, too young to choose their mother tongue, for a conflict being fought out between French- and Dutch-speaking adults tussling for political control of Belgium.

''This is linguistic wickedness,'' he told La Capitale newspaper. ''It is not fair and affects only French-speaking kids. The school's decision is dangerous.''

Two-fifths of the school's pupils are French-speaking and their parents have, since Christmas, been told via letter the children can no longer speak the same language they use at home during breaks.

''You have chosen a Dutch school. For children who speak another language at home there is no easy way out,'' the letter said. ''Dutch is compulsory at our school, even in the playground. If your child speaks French there, he will get a language slip. If he gets three, he will have to attend language classes after school.''

Veronique Vanhercke, the principal of Sint-Pieterscollege, insisted the measure was necessary because too much French was being spoken in the playground, undermining efforts to keep the school Dutch-speaking.

''At enrolment, parents committed themselves to support the efforts of their children to learn Dutch,'' she said. ''Our teachers were no longer taken seriously. I'm convinced that many other Dutch schools in and around Brussels have a similar policy. We do not see it as a punishment.''

Until last December, squabbling between the Flemish (Dutch-speaking) and Walloon (francophone) political parties kept Belgium without a government for 19 months, a world record, and threatened to tear the country apart.

Despite having Dutch, French and German as official Belgian languages, Belgium enshrines jealously guarded ''linguistic communities'' in its federal structure, creating municipalities where public services can be delivered in only one of the three Belgian tongues.

Divisions between Wallonia in the south of Belgium, and demands for more independence by Flanders, in the richer north, have been polarised in Brussels amid hostility to an influx of French-speakers into Flemish suburbs such as Jette, where the school is based.

Belgium's fragile coalition government is led by Elio Di Rupo, a Walloon Socialist of Italian heritage who is the first French-speaking prime minister of Belgium in almost 40 years. On taking office, he had to promise he would ''work'' on language lessons because, controversially, he does not speak Dutch.

Telegraph, London