An environmental task force of eight federal agencies led by the Interior Department recommended yesterday against construction of a giant pipeline to bring Alaska's natural gas reserves to the lower 48 states.

The recommendation, contained in an 89-page report detailing the environmental hazards of three proposed pipeline routes, was the first public evidence of apparently growing skepticism about the project within the Carter administration.

Should a pipeline be built, however - and it is still a strong possibility - a route proposed by Alcan Pipeline Co. partly paralleling the newly opened oil pipeline and veering east through Canada has the strongest backing from federal agencies.

The $8.5 billion project was examined by 16 federal agencies organized into 10 task forces. Each released a report yesterday on various aspects of the pipeline, which would transport 22 trillion cubic feet of Alaskan gas - equal to the nation's total consumption in a year. The Interior Department task force was the only one to recommend against the project.

President Carter is required by law to make a decision on the pipeline by Dec. 1, which Congress may overturn by joint resolution within 60 days. However, the fate of the proposal also rests with the Canadian government, which is expected to make up its mind this summer.

The pipeline's political, economic, environmental and foreign policy implications have stirred fierce controversy in Congress, in government agencies and around the country over the last three years. If it is built, it would be the largest private undertaking in the nation's history, according to the Federal Power Commission - more expensive even than the oil pipeline.

A task force led by the Federal Energy Administration said the $4,800-mile Alcan route would provide the highest economic benefits. Alcan is a group of Canadian and American companies led by Northwest Pipeline Corp.

In a May vote, the Federal Power Commission split 2 to 2 over the Alcan route and a rival 4,175-mile route through northeast Alaska and Canada proposed by Alaskan Arctic Gas Pipeline Co. This is a consortium of Canadian and American companies including southwestern, midwestern, and West Coast utilities.

A third route proposed by El Paso Alaska Co., a unit of El Paso Co., would build the pipeline across Alaska and transport the liquefied gas by tanker to California.

To a question about whether foregoing construction of any pipeline was a fourth option, a high administration energy official, who declined to be named, told reporters yesterday "We have considered it in the past, but are not now. If we don't, we will have problems with oil production on the North Slope." Gas and oil are mixed in the Prudhoe Bay Field, and experts fear oil production will be reduced if the gas is not also extracted.

Energy chief James R. Schlesinger would not discuss the pipeline issue on the record at a background briefing for reporters.

A joint Interior Transportation Department task force said the Federal Power Commission had sharply under-estimated the project's cost overruns and construction delays. Overruns could amount to 30 to 55 per cent, bringing the project cost up to $15 billion, it said, while the gas would sell at a competitive price of $2.50 per thousand cubic feet no matter who builds the line.

With estimated construction delays of 15 to 17 months, the task force said, the Alcan proposal could be finished first, in July 1984, while Arctic Gas and El Paso would finish in early 1985.

A task force led by the Treasury Department concluded that the pipeline could be privately financed - without federal assistance - if complex issues of price-selling, sales contracts and investment are resolved.

Although the companies are seeking federal guarantees, the task force recommended that consumers, the state of Alaska, the pipeline companies and the gas producers (Exxon, Atlantic Richfield and Standard Oil of Ohio) share the financing burden.

An Interior FEA task force on supply and demand projected the Alaska gas amounting to 5 per cent of U.S. consumption when it becomes available. It would result in a large reduction in predicted shortages, and reduce potential oil imports, the task force said.

Although some administration officials see Mexican gas as an alternative to building the Alaska line, the report said Mexican imports, offshore gas, coal gasification and other sources will be needed in addition to the Alaska supply by 1990.

Should the pipeline not be built, however, the report said Alaskan gas could converted to methanol, ammonia or urea products, or possibly transported by ice-breaking tankers.

Both Interiors environmental report, and a report issued yesterday by the President's Council on Envrionmental Quality's Council on Environmental Quality, said the Arctic Gas proposed would have the worst environmental consequences of the three. Its pipe would traverse the 8.9 million acre Arctic National Wildlife Range, an untouched wilderness.

"As our remaining national frontiers, lands in Alaska and Canada have a special charisma with a strong emotional pull even on peope who may never see them," the report said. "The ecological data base is incomplete. Many prime environmental values cannot be quantified."

The El Paso route across Alaska would "present serious potential for envrionmental degradation," the report, said because its facilities to and then regassify it could harm commercial fisheries by thermal and chemical pollution.

The El Paso line might also pass through the Chugach National Forest, a valuable and earthquake-prone wilderness, the report said.

Although the Alcan line was deemed the most environmentally acceptable, it too was found to present problems. Summer construction could damage the Arctic permafrost, the task force said.