MOSCOW, FEB. 16 -- British Foreign Minister Geoffrey Howe today said that London is not ready to consider reducing its nuclear arsenal even if U.S. and Soviet strategic stockpiles are cut by 50 percent.

Howe, on the second day of a visit to the Soviet Union, also brushed aside Soviet objections to NATO's plans to modernize some of its nuclear weapons.

In response to the Soviet charges that NATO is threatening the spirit of the new U.S-Soviet treaty to eliminate medium-range missiles with its modernization plans, Howe said at a press conference here that "modernization is not a euphemism for circumventing the INF {Intermediate Nuclear Forces} treaty."

A senior British official said today that Howe had said in extensive talks with Soviet Foreign Minister Eduard Shevardnadze yesterday that NATO reserved the right to modernize all weapons systems not covered by the INF treaty, including battlefield weapons, submarines and planes. He told Shevardnadze that areas not covered by the treaty should "not be cobwebbed into it," the official said.

Under the treaty, signed in Washington last December, the United States and the Soviet Union agreed to remove all of their ground-based missiles with ranges of 300 to 3,500 miles from Europe over a three-year period.

Asked in his press conference about reports that Britain is planning to deploy U.S. F111 fighter-bombers to compensate for the warheads to be removed under the treaty, Howe said, "no decisions have been taken on these matters." But, he added, "the deterrent sources available need to be kept up to date."

During a trip to Bonn last month, Shevardnadze objected in particular to NATO's Lance missile modernization program, but during two days of talks with Howe, Kremlin officials voiced their opposition to NATO's planned compensatory measures in more general terms.

In a dispatch released tonight, the official news agency Tass quoted Communist Party leader Mikhail Gorbachev as telling Howe that it is "impossible to explain the latest wave of praise for the 'nuclear deterrence strategy,' plans for so-called 'compensation,' all manner of projects for the joint manufacture of newest weapons, the Franco-West German military rapprochement, and so forth."

Appearing before journalists after a lengthy meeting with Gorbachev today, Howe gave a positive assessment of his talks and of British-Soviet relations.

"There is no doubt that . . . the significant improvement in British-Soviet relations has been maintained," he said.

But Gorbachev complained during the meeting of Britain's lukewarm response to several Soviet proposals, such as a ban on chemical weapons and reductions in conventional arms, according to Tass.

Gorbachev also dismissed Howe's pleas for increased emigration from the Soviet Union, Tass said, suggesting that such matters should be dealt with on a lower level, particularly in light of the steps Moscow has already taken.

A few days before U.S. Secretary of State George P. Shultz is due here for high-level talks on a proposal to reduce strategic nuclear weapons, Howe stressed that the British nuclear deterrent will play no part in the Soviet-American negotiations.

"If there would be very substantial reductions of the superpowers' offensive forces and if there would be no change in the defensive capability of the Soviet Union, then we would be ready to consider whether we could play any part," Howe said.

"But," he added, "we do not believe that situation would arise at a reduction of 50 percent."