Children as young as eight can be given the antidepressant Prozac, the European Medicines Agency (EMEA) has ruled.
The EMEA said that the drug was safe for young people to take, despite concerns that it can trigger suicidal feelings in patients.
However, the regulator's ruling said Prozac should only be given to children with moderate to severe depression who have not responded to several sessions of psychological therapy. The drug should also be given in small doses and must be used alongside counselling.
Mental health charities gave a cautious welcome to the ruling, saying that some children do need to be given Prozac. They are concerned that the drug may be prescribed more than necessary because of the long waiting lists for psychotherapy and other counselling services on the NHS.
Most antidepressants are banned or restricted for use among children after findings in 2003 that they can trigger mood swings and increase the risk of suicide among patients under 18.
One study found that 3.4 per cent of children experienced suicidal thoughts when taking antidepressants compared with 1.2 per cent who took a dummy, placebo pill.
Prozac was considered to be the only drug that could be safe among children and the British government last year asked the makers, Eli Lilly, to submit an application to the EMEA for its use among under 18s.
The EMEA review of all the data concluded: "Overall, the benefits of Prozac are greater than its potential risks for the treatment of moderate to severe major depressive episode in children and adolescents."
It recommended that children start on a small dose of 10mg of Prozac a day, which could then be increased to 20mg after one or two weeks. Only patients who have not responded to at least four sessions of psychological therapy should be prescribed the drug and if no benefit is seen after nine weeks the treatment should be reconsidered.
Doctors and parents should also be told to monitor children carefully for signs of suicidal behaviour.
The agency called for research into whether Prozac interferes with the sexual development of young patients.
Mental health experts said some children did need prescription drugs to treat depression, but warned that the lack of alternative therapies meant that many will be handed antidepressants inappropriately.
Dr Trevor Turner, the vice-president of the Royal College of Psychiatry, said: "The best kind of treatment uses a combination of cognitive therapy with some medication for a while, but I can imagine there will be a temptation to hope they can get by on Prozac alone because there are waiting lists for cognitive therapies."
Patients under 18 have to wait an average of eight months before they even have an assessment by their local child and adolescent mental health services and 10 months to see a child psychiatrist.
Children are written more than 85,000 prescriptions a year for antidepressants that are not recommended for use among young people, such as Seroxat. An estimated 40,000 young people under 18 are thought to be on antidepressant medication.
Avis Johns, the development director of the Young Minds charity, said: "There may be some circumstances where psychological treatment alone is not effective for the child.
"After careful consideration and consultation with the child, their family and medical team, such treatments may be offered alongside other therapy. However, it should never be the first course of action."
She added: "The impact of severe depression on the child, their family and wider community can be devastating. Although quite rare, the ability to combine a range of therapies with appropriate medication can provide significant benefits and should therefore be welcomed."