Portland must delay its effort to fluoridate Bull Run water until getting city voters’ approval.

The city elections office on Thursday verified that a referendum challenging the City Council’s September fluoridation decision has qualified for the ballot.

That puts an automatic stay on any efforts to proceed. A vote will be scheduled in May 2014, unless the City Council agrees to call a special election sooner than that date, says city elections officer Deborah Scroggin.

The city determined that Clean Water Portland submitted 33,015 valid signatures from registered city voters, well more than the 19,858 signatures required to refer a City Council decision to a public vote. Critics gathered the signatures in only 30 days.

“We are very excited about the strong public opposition we’re seeing to water fluoridation and it seems that Portlanders agree that adding more chemicals to our drinking water does not make sense when there are so many more effective ways to protect our children’s health,” said Rick North, of Clean Water Portland, in a prepared statement.

In a related decision, Clean Water Portland won a court challenge this week that will enable the group to proceed with a second ballot measure drive, an initiative to amend the Portland City Charter to ban fluoridation.

For that effort, the group needs about 30,000 valid signatures, but has more than a year to gather the petitions. The initiative would appear on the May 2014 ballot.

Portland voters are likely to get confused about the two signature drives and ballot measures on the same topic.

The referendum forces a public vote on the city’s decision, and voters can uphold or negate the decision. An initiative, if successful, would bar the city from ever fluoridating its water supply, until the City Charter is amended again.

“The referendum puts the genie in the bottle, and the initiative keeps it there,” said Kim Kaminski, director of Clean Water Portland.

Upstream Public Health and the coalition it formed, the Everyone Deserves Healthy Teeth Coalition, will lead the campaign against the referendum. Upstream was the group that mounted an under-the-radar lobbying campaign to convince the City Council to approve fluoridation.

“When Portlanders are provided with accurate information, they will be able to differentiate between myth and fact and will land on the side of water fluoridation because it’s good for community health,” said Raquel Luz Bournhonesque, co-director of Upstream Public Health, in a prepared statement.