South Central Strategic Health Authority (SHA) faced down a High Court challenge to its fluoridation plans a year ago but campaigners have found fresh grounds for dispute.
Hampshire Against Fluoride says the scheme could fall foul of the Water Act as Southern Water can’t guarantee the fluoridated water would only be distributed in areas defined in the initial proposals.
That means residents who haven’t been consulted on the plans could receive fluoridated water.
A spokesperson for the SHA said it remained confident with the decision to fluoridate and was working closely with Southern Water on technical feasibility planning.
The project is funded by Southampton City Primary Care Trust.
A Southern Water spokesperson said: “By law, if requested by a Strategic Health Authority to add fluoride to the water supply, then we must do so.
“We cannot take a medical or ethical view on the subject but are very mindful of our customers’ views.”
Currently, around 10 per cent of the UK population receives fluoridated water, which proponents say is a cost-effective way to reduce tooth decay.