The Reagan administration has been withholding approval of a proposed shipment of 231 pounds of plutonium -- enough to build a dozen atomic bombs -- from France to Japan aboard an ocean freighter because of concern over the lack of an adequate security plan, government sources said yesterday.

The Japanese proposal, which called for the U.S.-origin plutonium to be transported above-deck on a British containership during the 45-day voyage, provided for the shipment to be accompanied by only one unarmed security agent and did not include a plan to deal with an emergency, the sources said.

"This is an important first shipment of its kind since no American-origin material of this quantity has moved before, which is why it's so important that the physical security meets our requirements," Deputy Assistant Secretary of State James B. Devine said yesterday. "There will be other shipments like this in the years ahead and we want to make sure the precedents are good."

The plutonium is at the French nuclear reprocessing facility at LaHague, where it was separated from spent fuel generated by Japanese atomic power plants. Since the enriched uranium fuel that went into the Japanese power plants originated here, the United States retains the right to approve any subsequent arrangements involving the plutonium.

Japan is seeking return of the plutonium for use in Joyo, an experimental fast breeder reactor that has been operating since 1977, according to Tetsuhisa Shirakawa, first secretary of the Japanese Embassy.

"We would like to get this plutonium as soon as possible," Shirakawa said yesterday.

State Department sources said that although the administration is willing to approve shipment, the United States is "still some distance from working out" a security plan adequate to safeguard the plutonium.

Government sources said the Defense Department, the Arms Control and Disarmament Agency and the Nuclear Regulatory Commission are working with the Japanese to formulate a suitable security plan.

"We're probably talking about several more months before work is completed," a source said yesterday.

The Japanese proposal, sources said, called for the plutonium to be shipped from LaHague by truck to Cherbourg, where it would be transported by ferry across the English Channel to Britain. It would then be loaded aboard a containership owned by Overseas Cargo, Ltd. for the 45-day voyage to Japan.

The route, sources said, would take the plutonium around the tip of South Africa.

The lack of adequate contingency planning for a possible terrorist attack or other emergency has aroused concern among several members of Congress.

Rep. Richard L. Ottinger (D-N.Y), who earlier this year sponsored a resolution aimed at blocking the spread of plutonium, said the planned shipment to Japan "underscores the administration's cavalier attitude about putting nuclear explosives into world commerce."