UNITED NATIONS — The contest to be the next leader of the United Nations turned more complicated on Wednesday when a Bulgarian diplomat long rumored to aspire to the position formally entered the race.

The candidacy of the Bulgarian diplomat, Kristalina Georgieva, who is a vice president of the European Commission and a former World Bank official, could alter the diplomatic calculus between the West and Russia over who replaces Ban Ki-moon as secretary general in January.

It was announced in Sofia, the Bulgarian capital, by the government of Prime Minister Boiko Borisov.

Georgieva’s candidacy puts her in direct competition with a fellow Bulgarian, Irina Bokova, the director general of UNESCO. Bokova has fared poorly in five informal straw polls of the Security Council, which selects the secretary general.

Bokova said in an e-mail message Wednesday that she would remain in the race. But Borisov, speaking to reporters in Sofia, said that Bokova no longer had the Bulgarian government’s backing.

“We gave her one last chance, awaiting the results from the latest vote,’’ he said, referring to an informal straw poll taken Monday. He said the government now considered Georgieva “to be a more successful nomination.’’

Ban, a South Korean statesman, will complete his second five-year term at year’s end. The selection of his successor is ultimately in the hands of the five veto-wielding members of the Security Council, particularly Russia and the United States. The two are sharply divided, especially over the Syria war.

Twelve candidates have entered the race, and António Guterres, a former prime minister of Portugal and former head of the UN refugee agency, has emerged as the clear front-runner in straw polls. But diplomats say Georgieva is tough competition for him.