Russian billionaire Mikhail Prokhorov's Onexim company is close to agreeing the sale of a stake in Polyus Gold, and a potential deal is being reviewed by the U.K. Takeover Panel, two sources familiar with the matter said on Thursday.

Prokhorov's Onexim holding company owns 38 percent of the country's biggest gold company Polyus, which at the close of trading yesterday was valued at $3.6 billion.

In September, Polyus Gold said Onexim was in talks with two potential buyers to sell its stake in Polyus.

The identity of the buyers was not disclosed, but it will likely be two Russian investors who are not connected to the sector, one of the sources said.Prokhorov, who challenged Vladimir Putin in the last presidential elections, said in October that he is stepping back from his businesses in order to dedicate himself to politics.

Shares in Polyus rose eight percent to 216 pence.

Spokespeople for Polyus, Onexim and Prokhorov declined comment.