A coalition of nuclear scientists, arms control analysts and public health groups urged President Reagan yesterday to declare a moratorium on production of plutonium for nuclear weapons and seek a permanent production cutoff in forthcoming arms negotiations with the Soviet Union.

In a letter to Reagan, the groups said a production cutoff "would be an important cap on the arms race" and offer economic and environmental benefits to both nations.

The letter was signed by representatives from eight scientific and environmental groups and more than two dozen arms control experts and public health officials, including former Arms Control and Disarmament Agency director Paul C. Warnke, SALT I negotiator Ambassador Gerard C. Smith and former director of central intelligence William E. Colby.

Proposals to halt plutonium production have been discussed by arms control negotiators periodically for decades but have not been part of a formal U.S. negotiating position since 1969. At a news conference, drafters of yesterday's letter said the concept should be resurrected, in part because of newly documented dangers of continued operation of U.S. production reactors.

In a report last week, the National Academy of Sciences warned that the Energy Department's bomb production reactors in South Carolina and Washington state are deteriorating rapidly and may not be operating safely.

"The continued buildup of nuclear stockpiles threatens the health of America even if the weapons are never used in war," said Dr. Christine Cassel, president-elect of Physicians for Social Responsibility and a signer of the letter.

The Soviet Union's production reactors also are aging, the group said, and the problem there may be felt more keenly because of last year's disastrous nuclear accident at Chernobyl.

Princeton University physicist Frank von Hippel, research chairman for the Federation of American Scientists, noted that the Soviets expressed interest in a production cutoff at a special U.N. session on disarmament in 1982, even before Chernobyl heightened public awareness of nuclear dangers.

"That 1982 statement is still in force," he said. "We think the ball is in our court."

Advocates said that proposal also could forestall massive expenditures, estimated at $20 billion or more, for new U.S. production reactors that might not be needed if mutually verifiable arms reductions can be negotiated.

The coalition estimated that a two-year moratorium would reduce plutonium output by about 2,000 pounds, or 1 percent of the current stockpile.

Because most plutonium for new warheads is obtained from retired weapons, the group said, that reduction could be more than made up under the proposed intermediate-range nuclear force (INF) treaty that Reagan and Soviet leader Mikhail Gorbachev are to sign here next month.

The treaty could lead the United States to dismantle about 500 nuclear warheads, containing as much as 4,400 pounds of plutonium.

"The United States risks nothing and has much to gain" from a moratorium, the coalition said.

Spurgeon Keeny, president of the Arms Control Association and former deputy director of the ACDA, said yesterday that arms negotiators have long considered a plutonium-production cutoff attractive because of its relative ease of verification.

According to the coalition's letter, verification could be done by satellite and through the International Atomic Energy Agency, which monitors reactors in more than 50 nations, including two in the Soviet Union.

"I've thought for 25 years that this was a useful and doable constraint," said Keeny, who did not sign the coalition's proposal but

said he supports it. "It's an old

idea whose time may have come.

Some of the best ideas are the oldest."