LONDON (AP) _ Moscow will be incapable of a surprise attack on the West by 1991 if it makes the cuts in conventional forces promised by President Mikhail S. Gorbachev, the International Institute for Strategic Studies said today.

''The situation is already in the process of significant change,'' the respected institute said in its annual survey of global forces, The Military Balance 1989-90.

''The unilateral reductions will, once complete, virtually eliminate the surprise-attack threat which has so long concerned NATO planners,'' it added.

After the cutbacks, it would take the Soviets one to two weeks to marshal their forces for an attack, institute director Francois Heisbourg said.

''In shorthand, that translates as no surprise attack,'' Heisbourg told a news conference Wednesday. Soviet withdrawals under way include units needed in lightning attacks, he added.

In a speech to the United Nations on Dec. 7, Gorbachev promised to reduce the Soviet armed forces by 500,000 troops and to eliminate 10,000 tanks, 8,500 artillery pieces and 800 combat aircraft by 1991.

However, the institute said it has discovered - thanks to the Kremlin answering its questions for the first time - that the Soviet Union has 6,700 more main battle tanks than previously estimated, bringing the total to about 60,000. The institute said the additional tanks were probably in storage.

The Soviet-led Warsaw Pact's huge tank force is a key part of what the 16- nation NATO Western alliance says is the Communist bloc's overwhelming numerical superiority in conventional forces.

The institute's new survey showed NATO with 34,400 tanks compared with a Warsaw Pact total of 78,200.

Of these, NATO has 21,900 tanks deployed in the Atlantic to the Urals area, facing 58,500 Warsaw Pact tanks, the survey said.

Troop levels, excluding reserves, were shown as about equal in the same area, with NATO having 2.24 million active service personnel and the Warsaw Pact, 2.3 million.

The survey said the long-range strategic nuclear arsenals of the United States and the Soviet Union remained ''in rough parity.''

Institute officials said their estimates of conventional forces did not include Soviet cutbacks since Gorbachev announced the planned one-sided reductions.

Heisbourg gave no figures for withdrawals so far. But he said the Soviets have begun pulling out crack units and have announced that front-line divisions deployed in East Germany will be withdrawn.

''Some of these divisions are divisions which any NATO planner would like to see removed,'' said Heisbourg.

He cautioned, however, that the Soviets will still be capable of ''major sustained offensive operations'' even after the promised cutbacks.

He said the Warsaw Pact has been weakened by the political upheaval among its allies - Poland and Hungary implementing sweeping reforms and East Germany sticking to communism.

Both reformers and the hard-liners have maintained their military ties with Moscow, but relations are strained, said Heisbourg.

''It would be a very hardy analyst who would consider Hungary and East Germany still to be allies in any meaningful sense,'' he said.

The institute is an international, non-governmental center for research on security and arms control. It is funded by private foundations, membership fees and sales of its publications.