The incoming Reagan administration plans to accelerate development of a space-based laser weapon that could be used to defend against ballistic missiles launched from the Soviet Union.

The president-elect's Defense Department transition team has told key congressional Republicans that one of its top priorities is to increase spending on the high-energy laser system. It would fire an intense beam of laser light at ballistic missiles as they rose out of the atmosphere on their way to a target 5,000 miles away.

Reagan has told several Republican senators who will be taking over chairmanships of key committees that he would like to improve the nation's defenses against missile attack. Among the senators are John G. Tower (R-Tex.), incoming chairman of the Senate Armed Services Committee, and Harrison H. Schmitt (R-N.M.), the former astronaut who will chair the Senate subcommittee on science, technology and space.

"Gov. Reagan expressed a strong desire to expand our technical base to move the country away from the brink of nuclear war," Schmitt said in an interview. "In particular, he expressed a strong interest in the possibility of developing a laser defense against ballistic missile attack."

Pentagon officials briefing defense industry leaders in New York earlier this month said both this country and the Soviet Union are working on high-energy lasers that could be positioned in space. The lasers could be used not only as weapons against ballistic missiles but as orbiting fortresses in a space war.

Laser weapons have long been touted as a defense against ballistic missiles. But ground-based lasers that would fire at missiles descending toward targets have not proved practical. Laser light cannot penetrate clouds, fog, rain or snow, but in airless space there is no such difficulty.

In addition, a laser beam in space would not lose strength over thousands of miles. A laser weapon could be placed in a space-based battle station in a geosynchronous orbit 22,400 miles about the earth to match the Earth's rotation. Such a weapon would literally hover over one spot on Earth, such as a point in the Soviet Union.

Sen. Malcolm Wallop (R-Wyo.), a member of the Senate Intelligence Committee, has said he advocates stepped-up development of a space-based laser weapon in the new defense budget. Said Wallop: "There have been vigorous attempts to gain the support of President-elect Ronald Reagan and some advocates of this weapon are members of his new administration."

Ground-based test lasers have been used at Kirtland Air Force Base, N.M., to shoot down drone aircraft, and the Air Force has installed a turret-mounted laser in a KC135 airplane to see if it can shoot down air-to-air and surface-to-air missiles. It is not known whether these tests have been conducted.

At the present stage of development, laser weapons are at least 10 years from being used in space. Lasers tested so far as weapons are too large and unwieldy to be carried into space and left there. Even the space shuttle, which is due to make its maiden flight in March, is not big enough to carry these weapons into orbit.

Laser scientists hope speeded-up development will allow them to miniaturize laser weapons and their power sources so they could be put aboard satellites that weigh no more than 40,000 or 50,000 pounds. If that can be done, they could be hauled into space by the shuttle and deployed like robot battle stations.

A laser weapon would destroy a ballistic missile by striking it with laser light as it rose into low Earth orbit on the first leg of its journey to its target. It is then the most vulnerable to attack, since it is above the atmosphere and not behind cloud cover.