President Reagan's cherished "Star Wars" missile defense effort will be a far more modest endeavor at the end of his presidency than he and the research program's supporters once envisioned, congressional testimony and budget documents revealed this week.

Presented initially as a $5 billion to $6 billion-a-year research endeavor leading to deployment of a space weapons shield beginning in the early 1990s, the controversial program has recently been forced to lower sharply its ambitions and curb spending plans.

Managers of the project, officially known as the Strategic Defense Initiative (SDI), said that even if Congress accepts Reagan's proposed fiscal 1989 SDI spending plan, which most observers consider unlikely, the program will have spent 30 percent less than Reagan wanted since it began five years ago.

A key ambition of SDI supporters -- the testing of realistic antimissile weapons in space -- has been officially shelved for the remainder of Reagan's term, the officials said.

They added that research contracts are being canceled, deadlines stretched and some novel ideas are being cast aside as the program goes through "restructuring" caused by its lack of broad popular and congressional support.

According to Richard Bleach, SDI director of program planning, a decision to start deployment of the first phase of missile defense weapons will not be made before 1993, a year later than previously planned.

The delay stems in part from reduced funding. For example, Bleach said, the Pentagon sought $5.2 billion for SDI in fiscal 1988, but Congress appropriated only $3.6 billion. In fiscal 1989, the SDI Organization anticipated a $6.3 billion budget proposal, but Secretary of Defense Frank C. Carlucci has cut that by $1.7 billion, and Congress may cut it more.

In addition to the budget cuts, Bleach said, some research projects "haven't gone as planned."

Several officials explained that Carlucci decided that proposing more than a $1 billion SDI increase would be fruitless, given Congress' pattern of cutting earlier SDI budget proposals an average of 30 percent.

John Pike, a longtime SDI critic at the Federation of American Scientists here, called Carlucci's lower figure "a refreshing departure from the wildly inflated and clearly dead-on-arrival budget requests of previous years."

Bleach said Pentagon officials have been forced by the cuts to scale-back some experiments involving SDI sensors and rockets needed to detect, track and destroy Soviet ballistic missiles and nuclear warheads in space.

Longstanding plans for a $480 million space experiment involving a powerful neutral particle beam have been canceled in recent weeks, and construction of a powerful ground-based laser at White Sands, N.M., has also been delayed at least one year, other officials said.

Bleach also said "we have no money" in the proposed budget for fiscal year 1989 to conduct realistic space tests under the administration's disputed "broad" interpretation of the 1972 Antiballistic Missile (ABM) Treaty.

Former defense secretary Caspar W. Weinberger and other officials had long claimed such tests were necessary to maintain a schedule for beginning SDI deployments in the early 1990s. But Congress last fall barred the Pentagon from conducting such tests earlier than next October because they would conflict with a traditional or "narrow" reading of the ABM Treaty that was supported by the United States until 1985.

Although Congress' action left open at least a possibility that such tests could be conducted in the 3 1/2-month period between October and Reagan's departure, Carlucci ruled this out in testimony Thursday before the Senate Armed Services Committee.

"There are no tests scheduled for {fiscal year} 1989" that fall under the "broad" ABM interpretation, Carlucci said.

Effectively ruling out U.S. implementation of the "broad" interpretation in the immediate future, Carlucci also said two space tests planned for 1990 or 1991 "probably will not raise the treaty compliance issue."

Bleach explained that SDI managers were planning "to concentrate on {understanding} the technical feasibility" of SDI equipment through more ground-based testing and laboratory work, instead of elaborate space tests.

The most recent SDI space experiment, completed last week, cost $250 million and experienced several equipment failures.

Pike and other critics charged that SDI officials have responded to the budget shortfalls by taking funds from research on complex SDI weapons such as lasers and particle beams and lavishing it on less-sophisticated rockets needed at the outset of a potential SDI deployment.

They said this strategy created increased risks that the lasers might not be available when the Soviets have developed countermeasures to space-based rockets.

Bleach acknowledged that laser and particle beam research had been more deeply cut than rocket work, but he said the program managers were striving "to maintain a balance" between the near-term and far-term weapons.