California’s largest water agency voted Tuesday to add fluoride to the water it supplies to 18 million customers from the Mexican border to the central coast.

California, because of pockets of stubborn opposition, has long trailed the nation on water fluoridation. Only a handful of Southern California cities, including Beverly Hills and Long Beach, currently put fluoride in their water.

The Metropolitan Water District of Southern California said it will take more than two years to complete the massive project, which health officials called one of the largest such plans in the world. The agency supplies water to 26 cities in six counties.

“This is very significant in that public water fluoridation has been touted as one of the great public health achievements of the century,” said Dr. Timothy Collins, chairman of the California Fluoridation Task Force.

Collins, who also is head of dentistry for Los Angeles County, said fluoridation can reduce dental decay by 20 percent to 40 percent.

District director Ronald Gastelum said the measure will add less than $1 dollar a year to each customer’s water bill.

As of 1995, about 62 percent of the U.S. population was drinking fluoridated water, compared to only 17 percent of Californians, Collins said. Opponents of fluoridation in the state have cited health worries.