Teachers and nurses fear that the use of such drugs - for symptoms no more severe than youthful exuberance - is rocketing.

A United Nations’ panel found that 80 per cent of the world’s Ritalin, which is used to control hyperactivity, is consumed in the United States. School nurses in even small elementary schols are now known to administer daily doses of the drug to more than 40 children.

The investigation will examine the use of drugs such as Ritalin on children under six and will also consider whether the warnings on the medicines are adequate. Experts fear the development of young brains could be damaged.

A research paper in the respected Journal of the American Medical Association reports that prescriptions of Ritalin for two to four-year-olds tripled between 1991 and 1995. The use of Clonidine, Prozac and other pharmaceuticals used to treat depression and hyperactivity in children doubled during the same period, even though the manufacturers warn that they should not be given to children under six.

The number of under-fives taking Clonidine, used for treating attention deficit hyperactivity disorder, increased 28-fold.

“More children are being diagnosed with behaviour disorders, and more medication is being prescribed across the board, not just for pre-schoolers,” said Dr Daniel Safer, a co-author of the study nd an associate professor of psychiatry and paediatrics at John Hopkins University, Baltimore.

First Lady Hillary Clinton, who convened a White House conference on the matter, said: “Some of these young people have problems that are symptoms of nothing more than childhood or adolescence.” She also pointed out, however, that these drugs have “been a godsend for countless adults and young people”.

Two federal agencies have now begun investigations. The National Institutes of Health will begin a nationwide study of Ritalin use in children, while the Food and Drug Administration is considering more stringent warnings on the labels of psychiatric drugs for children under six.

Some critics contend that teachers are actually among those to blame, encouraging the use of the drugs to control undisciplined classrooms.

“Most disruptive behaviour among youngsters can be addressed through discipline and old-fashioned patience,” the Cleveland Plain Dealer newspaper commented. “Before adults resort to chemicals, they put time and effort into teaching children how to behave.”

However, a recent study of elementary schoolchildren, reported in Education Week and funded by the National Institutes of Health, found that Ritalin was more effective than behaviour modification therapy in treating attention deficit hyperactivity disorder.

The latest figures for the UK show Ritalin prescriptions for children rose from 3,500 in 1993 to 126,500 in 1998.