The UK's medicines regulator is examining a possible link between a swine flu vaccine given to millions of Britons and the sleeping disorder narcolepsy.

The Medicines and Healthcare products Regulatory Agency (MHRA) said it is "evaluating" case reports in Europe of patients developing narcolepsy after taking the drug Pandemrix.

Made by GlaxoSmithKline, the vaccine was the most widely used in the UK at the height of last year's flu pandemic.

On Friday, the European Medicines Agency said it was looking into Pandemrix after a number of cases of narcolepsy were reported, mostly in Sweden and Finland.

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 at least 30.8 million Europeans.

Narcolepsy is a rare sleep disorder which causes a person to fall asleep suddenly and unexpectedly.

In a statement, the European Agency said: "Although the cases of narcolepsy have been reported in temporal association with the use of Pandemrix, it is at present not known if the vaccine caused the disorder. The Agency's Committee for Medicinal Products for Human Use (CHMP) will look carefully at all of the available data to determine whether there is evidence for a causal association."

On Tuesday, Finland's National Institute for Health and Welfare recommended that vaccination with Pandemrix be stopped until the suspected link with narcolepsy had been thoroughly evaluated.

A spokesman for the MHRA said it was "aware of the case reports of narcolepsy" and was "evaluating these in collaboration with other EU authorities".

"Narcolepsy is a rare, natural illness, with around 10 new cases per million people every year and at present no link with the vaccine has been established," he added. "After use of more than six million doses of swine flu vaccine in the UK, no cases of narcolepsy have been reported following vaccination in the UK. Pandemrix vaccine remains available for use as recommended."