MOSCOW (AP) _ The Soviet Union will leave the orbiting space station Mir unmanned for three months because of delays in preparing equipment that cosmonauts need to perform experiments aboard the spacecraft, Tass says.

The news agency said Wednesday that cosmonauts Alexander Volkov, Sergei Krikalev and Valery Polyakov will return to Earth April 27, leaving the Mir station mothballed for only the second time since it was launched in 1986.

''When three Soviet cosmonauts return back to Earth from their orbiting station Mir in about a fortnight, the facility will be left unmanned for several months due to delays with the preparation of two research modules,'' Deputy Flight Director Viktor Blagov told Tass.

''This is because without the two modules, which will make it possible to conduct wide-ranging research, including studies in the interest of the Soviet economy, there is little sense in keeping cosmonauts on the space station any further,'' Blagov told the news agency.

Tass first announced Tuesday that the cosmonauts would return and leave the station without a crew, but the report Wednesday contained the first explanation of why the project was being suspended.

Some speculated the suspension was for economic reasons. The move was announced after critics called for deep cuts in spending on the space program. The announcement also came shortly after the Soviets lost two probes to Mars at a cost of nearly $480 million.

The mothballing of Mir makes economic sense, Blagov said. He said that expensive new equipment, both on the ground and on the space station, was needed, as well as training of future cosmonauts.

''It is fairly difficult to do this in the course of a manned flight, so the adjournment will simplify many things,'' he said.

The move, also announced on the evening television news program Vremya, effectively cancels the launch of cosmonauts Alexander Balandin and Alexander Viktorenko scheduled for April 19.

The announcement coincided with Cosmonauts Day, the anniversary of Yuri Gagarin's first manned space flight 28 years ago, which was marked Wednesday night with a seven-minute fireworks display that lit up the sky over Moscow.

Blagov defended the cost of the space program.

''We plan to promote space commercial services and have already begun to do this. Of the 19 agreements signed by the Soviet space agency Glavkosmos with other countries and foreign firms, 10 have been carried out,'' he said.

Blagov admitted that the Soviet Union was not capitalizing on space exploration. ''Soviet space technology is up to the highest world standards, but industrial spin-offs are pitifully scanty as very few of the space industry's achievements are turned over for commercial use,'' he said.

Mir, which the Soviets have said is the first building block of a permanently manned space station, has been mothballed only once before, a seven-month period following the return to Earth of its first inhabitants, Vladimir Solovev and Leonid Kizim, June 25, 1986, after three months in space.

Since the arrival of cosmonauts Yuri Romanenko and Alexander Laveikin in February 1987, the spacecraft has been manned.

On Dec. 20, cosmonauts Vladimir Titov and Musa Manarov set a world's record for space endurance, living aboard the Mir orbiter for 365 days and 22 hours.

They were joined in the final months of their record-setting space voyage by Polyakov, a physician who traveled to the Mir station on Aug. 28 to monitor the effects of living in a gravity-free environment for extended periods.

Titov and Manarov were replaced by Krikalev and Volkov. Polyakov stayed aboard the orbiter to continue experiments in space health techniques.