A petition with 14,000 signatures has been delivered to Downing Street by anti-fluoride protestors.
Campaigners are trying to reverse a decision by health bosses to add fluoride to tap water in Southampton.
Activists from Hampshire Against Fluoridation and other supporters handed in the petition on Tuesday.
The South Central Strategic Health Authority (SCSHA) said it was satisfied adding fluoride was a "safe and effective way" of tackling tooth decay.
It is the first time a health trust in England, rather than water companies, has been allowed to introduce fluoridation under new laws.
The board's decision was made in February despite 72% of 10,000 respondents in a public consultation opposing the move.
Opponents have voiced concerns over the impact it may have on people's health.
The government should do what they said they would do, which is only fluoridate where people are in favour
John Spottiswoode, chairman of Hampshire Against Fluoridation
John Spottiswoode, chairman of Hampshire Against Fluoridation, said the campaign had been forced "to go to the very top".
He said: "The government should do what they said they would do, which is only fluoridate where people are in favour and in Southampton they are very opposed."
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."
It added despite the views from the consultation an independent phone poll showed "no clear majority".
Last week, the Health Ombudsman said it may investigate the decision after receiving a number of complaints and could recommend the decision be overturned.
Southampton City Council was in favour, but Hampshire County Council was against the plan, which is designed to cut tooth decay and which will affect 200,000 people.
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