WASHINGTON (AP) _ Gen. Bernard Rogers, NATO's top military officer, told Congress on Wednesday that President Reagan's nuclear arms control proposal would cause problems for the western alliance because the Soviet Union has a large advantage in non-nuclear weapons in Europe.

Rogers, who is also head of U.S. military forces in Europe, said adoption of Reagan's proposal first made two months ago at the Iceland summit would reverse seven years of improvements in the forces of the North Atlantic Treaty Organization.

''It would put us back to where we were in 1979'' before NATO decided to upgrade its atomic forces with new Pershing 2 and ground-launched cruise missiles, Rogers told the House Armed Services arms control panel.

Rogers' criticism was not as strong as the comments he voiced shortly after Reagan's October summit meeting with Soviet leader Mikhail Gorbachev.

At that meeting, Reagan proposed the elimination of all nuclear-tipped ballistic missiles within a decade and the two leaders tentatively agreed to the elimination of all medium-range nuclear weapons, including the Pershings and cruise missiles, from Europe, according to the White House.

Agreement on the overall package, including the European reductions, fell apart because of disagreement between Reagan and Gorbachev over Soviet demands for limits on the U.S. ''Star Wars'' anti-missile program.

The U.S. proposal was criticized in Europe, where NATO has long relied on the threat of using nuclear weapons to forestall an attack by the Soviets. The Soviets and their Warsaw Pact allies have a large advantage in most categories of conventional weapons, such as airplanes, tanks, and artillery.

The 1979 decision to upgrade NATO nuclear weapons was highly controversial in Europe. The weapons are being installed now.

Rogers said NATO leaders ''concluded that we would be very much in favor'' of eliminating medium-range nuclear weapons from Europe.

But when Rep. Les Aspin, D-Wis., chairman of the panel, asked Rogers if the elimination would leave NATO militarily better off or worse off, Rogers said the decision ''would put us back to where we were in 1979 and in some aras, it would make us worse.''

Those areas include some types of aircraft and artillery, he said.

''In the equation of deterrence, the nuclear factor is the major factor,'' he said. The threat of nuclear reprisal is what creates the ''incalculable risk'' in the minds of Soviet leaders which keeps them from attacking NATO, he said.

Rogers also predicted that if Great Britain's Labor Party ever gains power and follows through on its threat to dismantle that country's nuclear forces, ''it would lead to the unraveling of the alliance'' because other NATO nations would also follow Britain's lead.

''My instincts tell that the United States would say, 'that's enough,''' he said, and withdraw the 326,000 American troops now stationed in western Europe.

But Rogers also said he had met with Labor leader Nile Kinnock and ''I found him a very pragmatic man,'' suggesting that if Kinnock is eventually elected prime minister, he might change his mind and retain the British nuclear forces.