Late last year President Reagan ordered work to begin on a chemical laser weapons program even though he believed -- according to his statements at a secret Oval Office meeting -- that it would violate the 1972 Antiballistic Missile (ABM) Treaty with the Soviet Union.

Dale Van Atta has spent more than six months confirming this story with knowledgeable, though reluctant, officials in the White House, the Pentagon and the intelligence community.

The president ordered Lt. Gen. James Abrahamson, head of the Strategic Defense Initiative (SDI) Organization, to proceed with the secret program, code-named "Zenith Star," despite its treaty-breaking implications.

The first phase of the laser project was a five-month study that began last January. The contractors were Lockheed Missiles and Space Co., Martin Marietta and Rockwell International.

The second phase is a three-month, $10.8 million effort to design a fully integrated space-based laser weapon for testing. An Oct. 5 contract made Martin Marietta Denver Aerospace the primary contractor, with subcontracts to Lockheed and TRW of Redondo Beach, Calif.

The program's intent was the most sensitive topic of discussion in the Oval Office meeting last Dec. 17, according to sources. Defense Secretary Caspar W. Weinberger arrived for a meeting at 11 a.m.; Abrahamson accompanied him to give a special SDI briefing.

While a variety of SDI programs were discussed, accounts of the secret meeting indicate that the key question was whether to move ahead with the alpha-hydrogen-flouride laser, under development by TRW since 1980 and now part of the project.

At the meeting, Weinberger said the chemical laser was the most mature "directed-energy" program under development, and said Zenith Star wast he best bet for an early demonstration of such exotic weapons. The president agreed.

But Reagan noted that Zenith Star would violatethe ABM Treaty as he understood it. Despite his stated view, Reagan said it was imperative to go ahead with the program.

At this point, according to sources, Abrahamson urged the president, in effect, to give the Soviets the six months' notice required by the treaty if either side intends to abandon it. Reagan tabled the suggestion for later high-level discussion.

The sources said Abrahamson argued that Zenith Star "should be done openly." The president disagreed. He explained that the political price he'd have to pay, with Congress already at his heels over the Iran-contra scandal, "would be too high."

Attempts by Van Atta to meet with either the president or chief of staff Howard H. Baker Jr. to discuss the story have been rebuffed for months. Reagan, Baker and communications director Thomas C. Griscom did not respond to Van Atta's most recent letters.

In an AUg. 14 interview with Van Atta, Abrahamson called his discussions with the president "a privileged area." He explicitly denied what our sources told us: that Reagan had ordered him to go ahead with the program after expressing his belief that it would violate the ABM Treaty.