Backed by environment and industry groups, the Environmental Protection Agency is about to propose the first federal pollution tax, intended to make truck and engine manufacturers pay costs they avoid by not meeting clean air standards.

Companies would be allowed to make products that violate the standards, but the tax was decribed as giving them an incentive to bring their trucks and engines into compliance as soon as possible. Products that cause extensive pollution would still be banned.

The Associated Press obtained a copy of the EPA proposal, which grew out of an agreement in October among 23 representatives of environment, industry and trade groups. None of the parties has backed out, said an EPA spokesman who asked that he not be identified.

It is not known how much revenue the tax would raise or whether the proposal will be modified as the agency writes the regulations to put it into effect.

Economists in general long have favored "pollution taxes" as an efficient way to reduce pollution, but many environmentalists have argued that such taxes are "a license to pollute" by companies willing to pay the price.

David Doniger, an attorney for the Clean Air Coalition and a key member of the negotiating panel, said that before the talks manufacturers were concerned that their payments might be "outrageous."

And he said environmentalists would never have agreed if it had been a "gut issue" such as dangerous radiation. "We're not arguing over how safe is safe," he said.

Under the proposal, the tax would correspond to the amount of money it would cost a firm to bring its products into compliance with the standards. The tax would de-escalate as it grew, but Doniger said, "It was worth getting the first part steep, where we think most of the violations will be."

He estimated that 10 percent of the nation's new heavy trucks, of which 1.4 million were sold last year, would be penalized in the first year, 5 percent in the second year and 2 percent in the third, with "a few million dollars" collected each year.

Congress authorized such penalties in 1977 amendments to the Clean Air Act, in part to give technological laggards in the industry an incentive to comply with tough standards without forcing them out of business yet without giving them a market advantage over technical leaders.