The Environmental Protection Agency is on the verge of endorsing legislation effectively banning phosphate laundy detergents in eight states bordering the Great Lakes.

Phosphates, a nutrient, cause excessive growth of underwater vegetation, which dies and decays, depleting the lakes's oxygen.

EPA's endorsement, if approved by the Office of Management and Budget, would represent a major policy reversal on the controversial water pollution issue.

Despite a national outcry that Lake Erie and its sister lakes were "dying," the Nixon administration in 1971 decided to oppose a federal ban of phosphate detergets, proposing instead that cities build sewage plants capable of removing phosphates.

Since then, however, research has shown that the most advanced treatment plants cannot adequately control phosphate pollution. Before leaving office last year, former EPA Administrator Russell Train endorsed a phosphate ban in Great Lakes states.

"Tens of millions of Americans depend on the Lakes for drinking water and recreation," said Sen. Gaylord Nelson (D-Wis.), who introduced a phosphate ban bill. "There is general scientific agreement that phosphorus is the major cause in the acceleration of the natural aging process in the Great Lakes. There is no doubt that the major and significant controllable source of the phosphorus is detergents."

Nelson's bill, an amendment to the Water Pollution Control Act now under revision in Congress, would prohibit the sale of laundry detergents and household cleaning agents with more than 0.5 per cent phosphorus in New York. Pennyslvania, Ohio, Indiana, Michigan, Illinois, Wisconsin and Minnesota.

New York, Indiana and the city of Chicago have already enacted phosphate detergent laws, leading to significant improvements in water quality. A Minnesota law is now under court challage.

A recent report for EPA's Chicago office said states should give "urgent consideration" to a phosphate detergent ban, which "would result in an immediate reduction of the euthrophielation rate" of the lakes, and in the amount of sludge left over from sewage treatment.

Eutrophication is the aging of a lake, which, as oxygen is depleted, becomes unable to support fish and eventully turns into a swamp. It is caused by hyman sewage, by detergents and by fertilizers that drain off farmland.

Fertilizers are a major source of phosphorus, but the EPA report said that a detergent ban would eliminate about 50 per cent of the nutrient, and save governments a minimum of $41.5 million a year in costs of phosphorus-treating chemicals and sludge disposal.

The ban would affect only powder detergents since liquid products do not contain phosphates, a Nelson aide said, adding that companies have come up with suitable substitutes for phosphate detergents.

A spokesman for Procter and Gamble, one of the major detergent manufacturers, said, however, that a phosphate detergent ban "is the only way to save the Great Lakes. Eutrophication has increased in the last year. (By loading the lakes with phosphates) we're creating a long-term crisis. It takes 200 years for the lakes to flush out their water."

The five Great Lakes form the largest body of fresh water on the continent. They contain 20 per cent of the world's fresh water supply - over 65 trillion gallons. In recent decades, pollution from cities farms and industries along the lakeshores have virtually emilinated whole species of fish and resulted in large "dead" areas in the centers of the lakes. Sixty per cent of the bottom of Lake Erie is depleted of oxygen.

The Internation Joint Commission, a Canadian-American body which overseas water pollution problems, has recommended a phosphate detergent ban every year since 1970. But "each year the U.S. government has refused to take action . . . EPA has been unwilling to listen to its own experts," Nelson said in a February speech.

Now, however, EPA Legislative director Charles Warren said the agency is likely to endorse Nelson's bill within several weeks.