Interior Secretary James G. Watt, in a surprise policy shift, said yesterday that he will ask Congress this week to enact a bill to forbid mining and drilling in wilderness areas until the end of the century.

The announcement left some environmentalist groups stunned at what they saw as a reversal of the administration's position on wilderness development. Other environmentalists, however, voiced suspicions of what one called a "Trojan horse," containing new threats to the nation's wilderness.

Watt's statement came on "Meet the Press" (NBC, WRC) when the secretary was asked about his statements that he would like to open up wilderness areas to oil exploration.

"This week I will ask the Congress . . . to quickly adopt new legislation that would prohibit the drilling or mining in the wilderness to the end of the century," Watt said. He said there would be only "one exception: if there is an urgent national need, the president should then with the concurrence of Congress, be allowed to withdraw those few acres that might be needed to meet that national need."

Watt said in a telephone interview later yesterday that his proposal includes two deadlines within the next five years, after which no new wilderness areas could be designated or proposed.

Watt said that now, the law allows mining and drilling in wilderness areas until 1984, when it would be prohibited. Since the Wilderness Act was passed in 1964, no mining or drilling leases have been granted for wilderness areas.

Altogether, Watt said, there are 80 million acres of wilderness and 20 million more proposed. All would be covered under his proposed drilling moratorium.

The reaction from environmental groups was strong and mixed:

Tim Mahoney, Washington representative of the Sierra Club, said he felt the Watt proposal is a "Trojan horse" and that since a number of key Republicans, including Sen. James A. McClure of Idaho, chairman of the Interior subcomittee, had abandoned the administration position on developing the wilderness, Watt was now trying to head off the defection by offering a new deal.

Mahoney, noting the deadlines on wilderness proposals, said the deal amounted to "Watt saying I will protect wilderness from oil and gas drilling, in return for cutting off the designation of any new wilderness for ever and ever."

But William A. Turnage, head of the Wilderness Society, said, "We congratulate the administration. It is a victory for the American people and for wilderness preservation . . . It was Secretary Watt who first raised the threat to wilderness. This is a complete turnaround in administration policy." He asked, however, what would happen to wilderness land in the year 2000 and what will happen to any new land proposed for wilderness.

Marion Edey of the League of Conservation Voters called Watt's announcement "incredible" and "even if there is some weird twist that makes the proposal different than it sounds , even if he just wants that kind of camouflage" of being pro-wilderness, it is an important sign, she said.

The league, coincidentally, is holding political strategy sessions with many environmental groups here this week. Edey said the group had just been told by pollster Patrick Caddell that there is strong public opposition to developing wilderness and that the issue could hurt the administration severely this year if it could be put to use by environmental groups.

Watt said in his telephone interview yesterday that the move "is a change in approach, but not in our goals. Our goal has always been to have the wilderness drilled or mined last . . . this is absolutely a change if you mean a change of tactics. But our goal has never changed."

He said the new proposal, besides protecting current wilderness lands against development, also would give Congress some deadlines on creating new wilderness areas: until the end of 1983 to act on lands now proposed for wilderness areas, and until the end of 1987 to propose any new wilderness areas.

After those deadlines, no new wilderness areas could be created, unless Congress made new law on the subject.

Watt's proposal also includes a provision destined to fuel an ongoing controversy. It would allow the secretary of the interior to release, for drilling and mining, any land that has been studied as a possible new wilderness area, but not officially recommended for preservation. Up to now, Interior makes recommendations whether to release the land, but Congress must review the recommendations before any leasing is done. Watt's proposal would allow Interior to lease land it has decided is not wilderness, before Congress agrees.

Watt said his action was prompted by the failure of his previous initiatives to pass in Congress and the need to act before the current wilderness act deadline.

The Wilderness Act of 1964 has a deadline of Dec. 31, 1983, after which wilderness lands will automatically be closed to mining and drilling. Watt said he had previously proposed extending that deadline for 20 years, to avoid a rush to drill and mine before the end of 1983.

Also in the proposal, Watt said, will be a new program to be undertaken by the U.S. Geological Survey to explore wilderness areas in detail to find out what oil, gas, minerals, and other resources exist in wilderness areas.

When asked what he meant by urgent national needs that would allow development of wilderness lands, Watt cited a severe energy embargo, or a national security requirement for such minerals as chrome and platinum, for which the chief suppliers are the Soviet Union and "South African states."