A possible increase in narcolepsy among children given the swine flu vaccine is being investigated by Finnish health officials.
They have found evidence of an increased risk of the disorder - in which people fall asleep without warning or sleep excessively in the daytime - among those aged four to 19 given the Pandremrix vaccine.
They say a preliminary study indicated there was an "observed increase in incidence of narcolepsy" among those given the vaccine.
The study, published by Finland's National Institute for Health and Welfare, said that "the most likely explanation for the findings is that the increase in narcolepsy is by joint effect of the vaccine and some other factor".
However, the Medicines and Healthcare products Regulatory Agency (MHRA), which regulates medicines in Britain, emphasised there was no confirmed link between vaccination and narcolepsy.
A spokesman said: "This signal has not been seen outside of Scandinavia and the exact reason why more reports of narcolepsy have been identified in Finland needs further scrutiny.
"As noted in the Finnish report, there has also been an increase in the number of narcolepsy reports in unvaccinated people in Sweden and Iceland, so a relationship with the vaccine is far from clear cut."
The Pandemrix vaccine remained available and should continue to be used as recommended, he said.
Children with health problems are among those people eligible to receive the Pandemrix vaccine, if doses of the seasonal flu jab, which contains a swine flu vaccine element, are unavailable.
GlaxoSmithKline, which produces Pandemrix, said it was aware of the Finnish report.
In a statement, the firm said: "This investigation is independent of a broader ongoing European Medicines Agency investigation initiated in 2010.
"GlaxoSmithKline is reviewing the report and believes it would be premature to draw any conclusions on a potential association between Pandemrix and narcolepsy until this European investigation has been completed."
The company said that more than 31 million doses of Pandemrix had been administered in 47 countries with 162 cases of narcolepsy reported in people who were vaccinated. Some 70pc of these cases were in Finland and Sweden, it said.