Fluoride in tap water can cause bone cancer in boys, a disturbing new study indicates, although there is no evidence of a link for girls.

New American research suggests that boys exposed to fluoride between the ages of five and 10 will suffer an increased rate of osteosarcoma - bone cancer - bet-ween the ages of 10 and 19.

In the UK, fluoride is added to tap water on the advice of bodies such as the British Dental Association. The Department of Health maintains that it is a cost-effective public health measure that helps prevent tooth decay in children.

About 10 per cent of the population, six million people, receive fluoridated water, mainly in the Midlands and north-east, and the government plans to extend this, with Manchester expected to be next. About 170 million Americans live in areas with fluoridated water.

The increased cancer risks, identified in a newly available study conducted at the Harvard School of Dental Health, were found at fluoride exposure levels common in both the US and Britain. It was the first examination of the link between exposure to the chemical at the critical period of a child's development and the age of onset of bone cancer.

Although osteosarcoma is rare, accounting for only about 3 per cent of childhood cancers, it is especially dangerous. The mortality rate in the first five years is about 50 per cent, and nearly all survivors have limbs amputated, usually legs.

The research has been made available by the Environmental Working Group (EWG), a respected Washington-based research organisation. The group reports that it has assembled a 'strong body of peer-reviewed evidence' and has asked that fluoride in tap water be added to the US government's classified list of substances known or anticipated to cause cancer in humans.

'This is a very specific cancer in a defined population of children,' said Richard Wiles, the group's co-founder. 'When you focus in and look for the incidence of tumours, you see the increase.

'We recognise the potential benefits of fluoride to dental health,' added Wiles, 'but I've spent 20 years in public health, trying to protect kids from toxic exposure. Even with DDT, you don't have the consistently strong data that the compound can cause cancer as you now have with fluoride.'

Half of all fluoride ingested is stored in the body, accumulating in calcifying tissue such as teeth and bones and in the pineal gland in the brain, although more than 90 per cent is taken into the bones.

MPs who have recently voted against fluoridation proposals in Parliament include Jack Straw, the Foreign Secretary, and Michael Howard, the Conservative leader.

Anti-fluoride campaigners argue that the whole issue has become highly politically sensitive. If health scares about fluoride were to be recognised in the courts, the litigation, especially in the US, could be expected to run for decades. Consequently, scientists have been inhibited from publicising any adverse findings.

The new evidence only emerged by a circuitous process. It was contained in a Harvard dissertation by Dr Elise Bassin at the Harvard School of Dental Medicine. The dissertation, completed in April 2001, obviously had merit because Bassin was awarded her doctorate.

However it has not been published. Environmental organisations were repeatedly denied access to it, and even bodies such as the US National Academy of Sciences could not get hold of a copy. Eventually two researchers from the Fluoride Action Network were allowed to read it in the rare books and special collections room at Harvard medical library.

Bassin told The Observer her work was still going through the peer-review process, and she hopes that it will then be published.

Dr Vyvyan Howard, senior lecturer in toxico-pathology at the University of Liverpool, has studied the new material.

'At these ages the bones of boys are developing rapidly,' he said, 'so if the bones are being put together abnormally because fluoride is altering the bone structure, they're more likely to get cancer. It's biologically plausible, and the epidemiological evidence seems pretty strong - it looks as if there's a definite effect.'

There is at present no understanding as to why males should be affected rather than females.

A Department of Health spokesman said that the latest evaluation of research in the UK had identified no ill effects of fluoride.