American and British scientists said yesterday they have developed a new nuclear breeder reactor and fuel processing system that can virtually eliminate the threat of atomic weapon proliferation.

Dr. Walter Marshall of the British Atomic Energy Authority and Dr. Chauncey Starr of the U.S. Electric Power Research Institute said their proposal would elminate the production of plutonium pure enough to be used in bombs.

At the same time the reprocessing of spent fuel from convectional reactors and the use of breeder reactors to make the world's supply of uranium last almost indefinitely.

Starr said the new proposal will be presented to President Carter, who banned the use of existing fuel reprocessing plants and halted the U.S. breeder reactor program because of weapon proliferation dangers.

Two environmental groups, New Directions and the Natural Resources Defense Council, issued a statement critizing the new proposal for failure to set high enough standards and saying the new technique could be used as a cover for bomb production.

Unlike conventional reprocessing methods which divide spent reactor fuel into three types of products - uranium, plutonium and radioactive waste - the new process would keep uranium, plutonium and some wastes mixed together in a substance so radioactive that it would kill anyone trying to steal it.

Spent reactor fuel elements would be reprocessed entirely by remote control, emerging from the concrete-enclosed chemical processing line as new fuel elements suitable for use in the breeder reactor.