Flu vaccines may not be as effective as people think, an expert has warned.

There is little clinical evidence that the vaccines have an effect on things like hospital stay, time off work, death in healthy adults or even those with conditions like asthma and cystic fibrosis, he said.

Vaccines given to children under the age of two have the same effect as if they were given a dummy drug, he added.

Tom Jefferson, co-ordinator of the vaccines field of the highly-respected Cochrane Collaboration, called for an "urgent" re-evaluation of vaccination campaigns.

Writing in the British Medical Journal, he said that, because influenza viruses mutate and vary from year to year, it was difficult for scientists to study the precise effects of vaccines.

Most studies are of poor quality, and there is little comparative evidence on the safety of the vaccines, he said.In addition, policy makers wanted to be seen to be doing something, all of which leads to a "large gap" between policy and the evidence, he said.

The difficulty in distinguishing between flu and flu-like illness added to the confusion, with some illnesses listed as flu when they were not, he said.

Earlier this month, there were fears that vulnerable people in the UK might have to wait for flu jabs after the Government confirmed stocks would arrive late.

The jab is given to high-risk groups first, including the over-65s, people with respiratory conditions such as asthma, and those with chronic conditions such as diabetes.