Phosphate detergents would be banned in the metropolitan area under a proposal endorsed by the Metropolitan Washington Council of Government's water board.

The board, which includes representatives of all area governments, backed a similar plan three years ago. This time, however, the ban has stronger support from local officials and utilities, and the COG staff has reported that its enactment would save at least $2.8 million yearly on sewage treatment. In addition, the Virginia State Water Control Board has dropped its long-standing opposition to the regulation.

As in the past, the nation's soap makers, through the New York-based Soap and Detergent Association, opposed the proposed ban, saying it would not result in the savings predicted by the COG staff. Association spokesmen also said if area residents were forced to use nonphosphate-based detergents, they would pay about $4.30 more each year for hot water and bleach.

But the COG staff, noting that 59 million Americans live with phosphate bans, said phosphorus concentrations in waste water have fallen 35 to 40 percent where similar prohibitions are in effect. Phosphorus is a nutrient essential to plant growth, but in heavy concentrations in waste water it can be a serious pollutant.

One of the major arguments against the ban has been the claim that detergents without phosphates don't clean as well. The COG staff said surveys have turned up little evidence of widespread consumer resistance, and noted that major soap makers have introduced a variety of nonphosphate detergents.

The vote last week by COG's water resources planning board was 8 to 1 in favor of the ban, with one abstention. The opposing vote was cast by Prince William County. Fairfax County's alternate abstained because the county's regular representative, Supervisor James M. Scott (D-Providence), was absent.

Any phosphate ban would have to be approved by all local jurisdictions.