Plans to add fluoride to tap water in Southampton have been rejected by Hampshire councillors, just hours after city councillors backed the idea.
Southampton City Council voted 26 to 18 in favour of the measure after a heated debate on Wednesday.
On Thursday, Hampshire County Council decided unanimously that it did not support the proposals.
The South Central Strategic Health Authority (SHA) will make the final decision after a 14-week consultation.
The city's primary care trust wants the level of fluoride increased in Southampton to cut tooth decay.
But county councillors concluded that more research and reassurances were needed before Southampton City Primary Care Trust takes further steps to increase levels of fluoride to one part per million.
Councillor Ken Thornber, leader of Hampshire County Council, said: "The Southampton City Primary Care Trust wants to improve the oral health of specific communities in Southampton, but their proposals will impact on people in south-west Hampshire which does not have the same problems of poor dental health.
"There may be some benefit to some children living in the affected area, but there is also a strong possibility that children with otherwise healthy teeth may develop a degree of fluorosis.
"It is not fully understood if there are other health effects to a population that has fluoride added to drinking water."
Earlier, Hampshire PCT put its weight behind the idea but said views of all residents across the county should be taken into account.
The county council will send its views to the SHA for assessment.
Opponents claim fluoride has negative effects on the body.
John Spottiswoode, chairman of Hampshire Against Fluoridation, said: "When most countries across Europe have tried water fluoridation and abandoned it as ineffective and dangerous, why should we in Southampton be forced to go down this scientifically discredited route?"
If the plans are approved more than 200,000 people would be affected.