Internet search engines are far more likely to guide people to sites which oppose the vaccination of children, according to a survey.

The top 10 results from the most popular search site, Google, all referred to anti-vaccination sites.

In the UK, the medical authorities have tried to defuse the current controversy over the MMR vaccine with an information campaign aimed at parents.

However, uptake of the jab is still missing official targets.

Many of them purported to give both sides of the argument, but very few of them did

Professor Simon Chapman

It is estimated that 55% of adults with internet access use it at one time or another to seek out health information.

Some doctors are concerned that "official" sites run by governments or expert groups have equal status with those set up by those with no medical qualifications.

However, the latest study, published in the journal Archives of Disease in Childhood, suggests that the pendulum has swung firmly in favour of non-governmental sites.

Word search

Researchers from the University of Sydney typed the words "vaccination" and "immunisation" into seven search engines, including Google, Yahoo, Lycos and Altavista.

In total, 43% of the links displayed led to anti-vaccination sites.

These were varied in content - including what the researchers described as "quasi-official" sites with referenced material from medical journals, to personal testimonies, "back to nature" appeals and sites alleging cover-ups and conspiracies involving doctors, governments and pharmaceutical companies.

The researchers said that perhaps the only way to attempt to refute the claims of those opposed to vaccination was to produce web information as emotive and powerful as on anti-vaccination sites.

Lead researcher Professor Simon Chapman told the BBC: "Many of them referred to the medical literature, but not in a way which was coherent or comprehensive.

"Many of them purported to give both sides of the argument, but very few of them did.

"The most disturbing thing is that a parent going to the web to search out information is going to have a very high probability of arriving at such a site.

"Some of them are extremely professionally produced, they look authorative, they have a lot of information on them, but unfortunately many of them are really very irresponsible."

Professor Chapman said the web had made information accessible to millions around the world, but it was very difficult to institute a system of quality of control.