Experts have indicated a possible link between the swine flu jab and an increased risk of developing a rare nerve disease.
The authorities are carrying out studies to examine a possible association between the vaccine and Guillain-Barre Syndrome, a condition that attacks the nervous system and can cause paralysis and even death, reports the Telegraph.
The Medicines and Health care products Regulatory Agency (MHRA) has published a report that suggests that further tests are to be carried out.
It reads, "Given the uncer-tainties in the available information and as with seasonal flu vaccines, a slightly elevated risk of GBS following H1N1 vaccines cannot be ruled out.
"Epidemiological studies are ongoing to further assess this possible association."
A vaccine used to combat a different form of swine flu in the US in 1976 led to 25 deaths from the condition, compared with just one death from swine flu itself.
Amid fears there could be a repeat, neurologists were asked to record cases of GBS in the UK swine flu outbreak.
Government experts say there is no evidence of an increase in risk similar to 1976, but the MHRA report reveals they are calculating if there might be a smaller raised risk.
A spokesman for the MHRA said the risk with the vaccine had not changed and that the report "simply expands" on ongoing GBS analysis.
"The position was and remains that there is no confirmed evidence that the vaccines are a cause of GBS," he said.