A flu vaccine given to millions of Britons, including children, is not suitable for those aged under 20, European regulators said today.
Pandemrix should only be given to children and teenagers if other vaccines are unavailable and they still need protecting against the H1N1 strain of flu, the European Medicines Agency (EMA) said.
It follows an investigation into reports from Finland and Sweden of children and adolescents suffering narcolepsy.
Narcolepsy is a rare sleep disorder which causes a person to fall asleep suddenly and unexpectedly.
The EMA said the overall risk benefit of Pandemrix remains positive but studies had shown a six to 13-fold increased risk of narcolepsy in children and adolescents vaccinated with Pandemrix compared with unvaccinated children.
The EMA said "the vaccine is likely to have interacted with genetic or environmental factors which might raise the risk of narcolepsy, and other factors may have contributed to the results".
Narcolepsy has not been confirmed but cannot be ruled out in other countries, it added. Some studies are still ongoing.
An increased risk has not been found in adults over the age of 20.
Made by GlaxoSmithKline (GSK), Pandemrix was the most widely used vaccine in the UK at the height of the 2009/10 flu pandemic.
The vaccine was given to millions of people in high-risk groups, including children and those with conditions such as diabetes, heart disease and asthma.
Overall, it was given to more than 31 million people worldwide.
A statement from GSK said 335 cases of narcolepsy in people vaccinated with Pandemrix have been reported to it as of July 6 this year.
It added: "GSK is committed to patient safety and will continue to work closely with the EMA and other national regulatory organisations in the best interest of patients.
"Further information from ongoing studies, including the final data from the VAESCO (Vaccine Adverse Event Surveillance and Communication) study and an epidemiological study in Canada being supported by GSK, is however still needed in order to gain additional insight into the cause of the reported cases of narcolepsy.
"In addition, GSK has committed to conduct further research into any potential association between Pandemrix and narcolepsy and will seek independent expert advice on this research activity, as agreed with the EMA."