Shares in the company fell earlier following a report in the Daily Mail newspaper that the company's Fluvirin vaccine contained the preservative thimerosal, sparking concerns about possible links with autism and brain damage.

The paper said that 12 million people in the UK, including children, were given flu shots this winter and that four out of seven vaccine brands used, including Fluvirin, contained thimerosal, which acts as a preservative against contamination.

Thimerosal, which has been used as a preservative in vaccines for decades, has not been scientifically linked with autism or brain damage, but vaccines with it are no longer used in children because of public sensitivity surrounding its use.

"Fluvirin sold in Europe has no mercury preservative in it," a company spokesman told Reuters.

Shares in the firm were down 1.81% at 396.10 pence in mid-afternoon trade in London.

Fluvirin--now the world's second biggest-selling flu vaccine--accounts for nearly 70% of the Oxford-based firm's sales, and its success has been instrumental in making PowderJect only the second British biotech company in history to post a profit.

The spokesman said PowderJect continued to sell Fluvirin containing the preservative in the United States, but even there it was not used in vaccinating children.

Since 2001, the firm also has been selling a preservative-free flu vaccine in the US.