The Soviet government has unexpectedly granted an exit visa to a prominent physicist who in recent months had become an important leader in Moscow dissident circles.

Sergei M. Polikanov, a 51-year-old nuclear researcher who last year broke with his privileged scientific colleagues to denounce offical restrictions on his personal and professional life, said yesterday that he will be leaving within two weeks for Denmark with his wife and daughter.

The scientist, an Order of Lenin winner and internationally known researcher into the composition and structure of the atomic nucleus, told Western reporters that he was called to the Moscow visa office last week and told his application to travel to Denmark would be approved. Polikanov had spent some time there in the 1960s, working at the Nils Bohr Institute.

Western diplomatic sources here said the permission was given because it removes from the depleted dissident group a vigorous new member who speaks excellent English and knows a number of Western journalists.

In July one day after the trial of Anatoli Scharansky ended, Polikanov joined the Helsinki monitoring group, which seeks to check up on Soviet compliance with the human rights guarantees of the 1975 Helsinki accord signed by the Kremlin. The physicist, his wife, Alexandra, and their 17-year-old daughter, Ekaterina, had become familiar figures outside the trials of Yuri Orlov and Alexander Ginsburg, cofounders of the Moscow monitoring group.

Polikanov surfaces last November as a critic of authorities who had barred him from working in Geneva, Switzerland, with scientists from Western countries who were doing experiments into areas pioneered by Polikanov.

Since then, he has been expelled from the Communist Party, of which he was a member for 22 years, and dismissed from his position as head of a research laboratory at the Dubna Atomic Research Center north of Moscow.

Polikanov said he intends to do research work in Denmark for a year and then will decide where else he might try to live. As a corresponding member of the Soviet Academy of Sciences, he enjoyed extraordinary prestige and privilege here.

The visas will allow the family to travel on Soviet passports, unlike many other dissidents who leave here as stateless persons.