WASHINGTON (AP) _ The label ''degradable'' is surfacing on plastic products from trash bags to diapers, but environmentalists call it a marketing ploy that should be rejected.

Six environmental groups called Tuesday for a boycott of all degradable plastic products, contending there is no proof that the products will decompose in a landfill any faster than conventional plastics.

''Degradable plastics are a consumer ripoff,'' said Jeanne Wirka of the Environmental Action Foundation, a Washington-based environmental group.

Richard Denison of the Environmental Defense Fund accused manufacturers of ''preying on the environmental concerns of American consumers'' to sell trash bags and other plastic items.

''Degradable plastics may be good for profits, but they are bad for the environment,'' Denison said.

In recent months, an increasing number of degradable products - from trash and grocery bags to diapers - have cropped up on grocery shelves. Manufacturers contend the products made of a plastic compound will degrade in a relatively short time when left to sunlight and, in some cases, even in landfills.

The products include Hefty and Glad trash bags and several brands of diapers. Some supermarkets also are increasingly using degradable plastic bags for carrying groceries home because environmentally concerned consumers shy away from conventional plastic bags.

Biodegradable plastics are made by adding corn starch to the plastic polymer.

Archer Daniels Midland Co., which manufactures the corn starch additive, maintains that its use will cause the plastic to degrade in a landfill ''within three to five years.''

''The life of current landfills can be extended by 20 percent while achieving an operational cost savings of 30 percent,'' company vice president Martin Andreas said recently when describing the degradable plastic product.

''They will not solve all our solid waste issues, but they have definite advantages if applied properly,'' said Tim Draeger, executive director of the Degradable Plastics Council, a St. Louis-based group made up largely of companies involved in the production or use of degradable plastics.

The plastics industry is divided on the issue.

''Degradability is not the solution to the solid waste problem,'' said Robert J. Barrett, general manager of solid waste management solutions at Mobil Chemical Co. in Pittsford, N.Y.

Mobil, which produces Hefty bags, turned to degradable bags because of consumer and competitive pressures, he said.

''We have found ourselves in the position of a strong consumer demand for degradable plastics'' and competitors are providing the product, Barrett said in a telephone interview.

He said the plastic in Hefty bags will decompose faster than conventional plastic when exposed to sunlight, but responds no different from other plastic when buried.

The plastic compound used by Mobil is different from the corn starch and plastic polymer mixture that makes up the degradable plastic products advertised as decomposing when buried.

Barrett said Mobil had no definitive proof that the corn starch mixture will decompose significantly faster in a landfill environment than conventional plastic.

The groups calling for the boycott are the Environmental Defense Fund, Environmental Action Foundation, Environmental Policy Institute-Friends of the Earth, Natural Resources Defense Council, Public Citizen and the Coalition for Recyclable Waste.