At least 12 countries have reported a possible link between a swine flu jab and a rare sleeping disorder, the World Health Organization has confirmed.
It said "further investigation was warranted" following reports of 52 cases of narcolepsy linked to the Pandemrix jab in Finland.
Cases have also been reported in Sweden, Iceland and the UK.
The government agency which regulates medicines in the UK said a link had not been confirmed.
The Medicines and Healthcare products Regulatory Agency (MHRA) said: "The Pandemrix vaccine remains available and should continue to be used as recommended.
"The benefits of vaccination outweigh any risk of a possible side effect."
Narcolepsy is a rare condition where a person falls asleep suddenly and unexpectedly.
A possible connection between the swine flu vaccine and the illness first came to light in Finland.
Finland and other Scandinavian countries have reported a general rise in cases of narcolepsy - in both vaccinated and unvaccinated people - but the cause has not been established.
A recent investigation by Finnish authorities found children given the swine flu jab Pandemrix were about nine times more likely to develop narcolepsy than those who had not been vaccinated.
The WHO's global advisory committee on vaccine safety reviewed this, and other data, last week.
It said the committee would continue to monitor the situation and agrees that "further investigation is warranted concerning narcolepsy and vaccination against influenza (H1N1) 2009 with Pandemrix and other pandemic H1N1 vaccines".
The WHO said an increased risk of narcolepsy had not been observed with any other vaccines - for flu or other diseases - in the past.
It said: "Even at this stage, it does not appear that narcolepsy following vaccination against pandemic influenza is a general worldwide phenomenon and this complicates interpretation of the findings in Finland."
The Pandemrix vaccine, made by Glaxo Smith Kline, has been used in 47 countries following the swine flu outbreak last year.
More than six million doses of the H1N1 vaccine have been given in the UK, with more than 30 million given across Europe.
The MHRA said it had received four unconfirmed reports of narcolepsy following vaccination.
It said the reports so far in the UK "are no more than we would expect to see by coincidence after vaccination".