It has warned schools which are flouting the law on single-sex education that their licences will be revoked if they do not comply immediately.

According to a 26-year-old law, classes from the fourth year onwards must be single sex and taught by teachers of the same gender.

But until now the law has rarely been enforced in private schools, particularly those catering for the children of western residents.

Some private schools say they may have to double fees to cover the cost of extra staff, classrooms, libraries and playgrounds.