Councillors are fighting Government moves which could lead to fluoridation of drinking water supplies.
The row will erupt in the House of Commons next Monday when MPs take part in the second reading of the Water Bill.
Brighton and Hove City Council is among authorities against the plans after peers voted in favour of smoothing the way for water companies to put fluoride in supplies when the Bill was debated in the Lords in the summer.
Dentists say adding the substance will help thousands of children grow up with healthy teeth.
The Government backs their view and will discuss amending the Bill in the Commons when Parliament resumes on Monday.
Under the plans, health authorities would be able to add fluoride to supplies if the public supports the idea.
Prompted by the Green Party, the city council has voted to oppose the move, because of concerns fluoride is toxic.
Green members put a motion before the full council in July, calling on councillors to write to Southern Water and health authorities to express opposition.
Keith Taylor, convenor of the council's Green group, said fluoridation would breach the European Convention on Human Rights and European Union rules on medical intervention.
He said: "Fluoride is more toxic than lead and only slightly less so than arsenic and it should not be forced on people."
Now there is growing unease about the proposal from some local councils, including Brighton, Bradford and Lancaster, which have all passed motions opposing the Government's plan.
Southern Water said fluoride was not added to the county's water supply, although some companies in the north of England do add it.
The company said it could not take a medical or ethical view on fluoridation and would want a clear message the public was in favour before agreeing to add the substance.
The Government asked the council for its views as part of its consultation on amendments to the Bill.
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