Grigori Romanov, one of the senior figures in the Soviet hierarchy and long regarded as a rival to Soviet leader Mikhail Gorbachev, was ousted today from all of his posts in a move that underscored Gorbachev's dominant position in the new leadership.

An official announcement said the 62-year-old Romanov was relieved of his membership in the ruling 13-man Politburo and also of his post as secretary of the Communist Party Central Committee "in connection with his retirement on health grounds." The announcement, distributed by the official news agency Tass, said the Central Committee had acted at Romanov's request.

There have been persistent rumors in recent weeks that Romanov's political future was uncertain. According to one version, which could not be verified, Romanov had sought to block Gorbachev's election to succeed the late Konstantin Chernenko last March.

At the time of Chernenko's death, only Gorbachev and Romanov were both Politburo members and secretaries of the Central Committee, a combination of jobs required for anyone aspiring to become general secretary of the party, the top leadership position.

The Central Committee, meeting on the eve of a biannual session of the Supreme Soviet -- the nominal parliament -- also elected Eduard Shevernadze, 57, to the Politburo.

Shevernadze, who had been an alternate or nonvoting Politburo member, is the Communist Party leader of Soviet Georgia. Before taking that job in 1972, Shevernadze, a three-star police general, had served for eight years as the Georgian interior minister.

The Central Committee also elected two new members of the Secretariat, which is the party's second most influential body after the Politburo. The new Central Committee secretaries are Lev Zaikov, 61, the Leningrad Communist Party chief, and Boris Yeltsin, the party chief of the Sverdlovsk region, one of the main centers of the Soviet military industry.

The election of Zaikov and Yeltsin brings to 10 the number of secretaries. Gorbachev, as general secretary, runs both the Secretariat and the Politburo.

The ouster of Romanov is likely to be seen as an ominous signal for those who fail to support the new leadership.

Romanov had held full Politburo membership since 1976. Since 1970, he had been the Communist Party leader of Leningrad, which ranks second only to Moscow as a political and economic center. He was moved to Moscow in 1983 and made a secretary of the Central Committee, the step that enhanced his standing in the Politburo.

Over the years, his name was linked to a series of indiscretions. One involved a wedding party for his daughter seven years ago for which Romanov borrowed Catherine the Great's dinner service from Leningrad's Hermitage Museum. After some of the historic china was broken in the revelry, he was privately criticized for behaving not as a communist but as a Romanov, the name of the last imperial dynasty.

Tass said the Central Committee today examined issues to be discussed at the session of the Supreme Soviet which opens Tuesday.

The Supreme Soviet was expected to elect a new president, since the post was vacated by the death of Chernenko. Most diplomats believe that Gorbachev will take over as titular head of state, but there has been persistent speculation that the post may be given to veteran Foreign Minister Andrei Gromyko.