Campaigners handed a petition with 14,000 names to Downing Street
A judicial review has been initiated by a resident against a decision by health chiefs to add fluoride to tap water in Southampton, a law firm has revealed.
Leigh Day & Co solicitors said it had begun the legal challenge on behalf of Geraldine Milner against South Central Strategic Health Authority (SCSHA).
The SCSHA instructed Southern Water to fluoridate the water despite their consultation showing major opposition.
It said it was satisfied it was a "safe way" of tackling tooth decay.
The SCSHA board's decision was made in February despite 72% of 10,000 respondents in a public consultation opposing the move.
It is the first time a strategic health authority in England, rather than water companies, has been allowed to introduce fluoridation under new laws.
As a result of the authority's move, activists from Hampshire Against Fluoridation and other supporters delivered a petition with 14,000 signatures to Downing Street on 9 June.
On its website, a statement from Leigh Day & Co says: "Ms Milner is opposed to the proposals to fluoridate the water supply on account of the continuing uncertainties with regard to the long-term health risks associated with fluoridation."
It also stated that she was concerned "with regard to the possible adverse environmental effects".
Leigh Day & Co's legal challenge argues that the "SCSHA failed to have regard to the government's policy that mass fluoridation of drinking water should only go ahead in any particular area if a majority of the local people are in favour of it".
It also argues that the SCSHA "failed to follow the requirements set out in the regulations to evaluate arguments in the responses to the consultation for and against fluoridation".
John Spottiswoode, chairman of Hampshire Against Fluoridation, said: "We fully support [Ms Milner] launching a judicial review."
Earlier, in a statement, the SCSHA said: "The board is satisfied that, based on existing research, water fluoridation is a safe and effective way to tackle tooth decay and that the health benefits outweigh all other arguments against water fluoridation."