Oil from the broken tanker Amoco Cadiz was drifting menacingly close to the British channel island of Guernsey yesterday, while on Jersey, to the south, authorities organized their defenses as the giant slick was reported shifting eastwards towards them.

"There is no doubt about it, we are in a risk condition," a Jersey Defense Committee spokesman said."We shall be working throughout the day to achieve a full alert by tonight."

He said the oil, pushed along by high winds, now was 40 miles south of Jersey between the island and the French coast. Officials reported the slick was about 35 miles from Guernsey, The Amoco Cadiz, an American ship with 1.6 million barrels of oil aboard, foundered March 16 on rocks near Brest, France.

"We are concentrating getting more spraying vessels congregated on Guernsey," a British Department of Trade spokesman said. "But the weather is not helping us at the moment."

He said authorities were building up reserves of chemical dispersants on the island, with tugs to maintain round-the-clock spraying operations.

"The intention is to have sufficient reserves on Guernsey so we can keep five or six tugs spraying continuously and fuelled up," he said. "As soon as one comes back to refuel, it is replaced by another one."

In Paris, Prime Minister Raymond Barre said that as of Monday, tankers will be prohibited from coming within seven miles of the French coast and would be obliged, on entering French territorial waters, to signal their position, their route and whether they were experiencing any difficulties.

Barre said the Amoco Cadiz wreck was caused by "grave negligence" on the part of the ship's captain and the captain of the West German tug that went to its aid.

Capt. Pasquale Bardari, 37, master of the tanker, was charged Monday with breaking French laws covering pollution by negligence. The captain of the tug has not been charged but the investigation is continuing.

More than 1.2 million barrels are estimated to have spilled out of the tanker so far. French officials are still considering whether to try to set the wreck on fire to destroy th remaining 350,000 barrels.

Harry Rinkema, Amoco's vice president for marine transportation, said the company was asked Wednesday to "consider and comment on controlled burning."

He said, however, that salvage experts think burning is "a very controversial subject. There are serious doubts if any burning under controlled conditions can be carried out efficiently and successfully."

Rinkema said the basic plan is still to try to remove the remaining oil by pumping, either through bringing smaller ships alongside or running a pipeline out to a tanker moved in safer waters when the weather subsides.

Local inhabitants and several experts fear that burning could cause a huge cloud of black smoke that would move inland and damage farmlands.

French officials said that as the oil slick moves north toward the Channel Islands, it is also posing a threat to Mont St. Michel monastery which is situated on a rock in a bay along the French coast.