Newcastle and North Tyneside health authority has lost a High Court bid to force Northumbrian Water to fluoridate the water of millions of customers.

The authority, acting on behalf of six authorities throughout the north east, has decided not to appeal but to seek changes to the Water Industry Act to put water companies under a duty to add fluoride.

Mr Justice Collins ruled in December that Northumbrian Water was under no statutory duty to put fluoride in the water supplies of its 2.5 million customers.

Forty per cent of customers in Newcastle, Gateshead and Derwentside already receive fluoridated water. The judge rejected a claim by the authority that the company's refusal to extend its fluoridation programme was 'unlawful and irrational'. He said the act merely gave companies a power to fluoridate, adding: 'Water companies are entitled to say no to fluoridation even if they have no reason to doubt it would be in the interests of the health of its customers or if its customers want it.'

The judge said it was for Parliament to change the law if it wished. The company suggested during the hearing that a mistake in the amount of fluoride added to water might open it up to vast numbers of compensation claims.

David Evans, consultant in dental public health for Newcastle and North Tyneside, said: 'We will be writing to the minister and [public health minister] Tessa Jowell to say now we have evidence that the act is not working, and the only way of progressing is by changing the act.'