BP paid $US375 million to the Alfa Group and Access-Renova to take its equity to 25% plus one share, a level of equity that matches its current voting rights.
BP staff currently hold the posts of Sidanko chairman and president as well as several other key executive positions. Analyst said the increased holding will further consolidate BP's influence in the company.
As for Alfa Group and Access-Renova, the two will remain majority shareholders in Sidanko following completion of the transaction.
The move is seen by industry watchers as evidence of a turnaround in the attitude of Western companies toward buying shares in Russian petroleum companies.
"This purchase underlines BP's confidence in Russia and its improving business environment," said BP chief executive, Lord Browne.
BP nearly lost all of its half a billion dollar investment in Sidanko when OAO Tyumen Oil Co took over Sidanko's largest oil fields in 1999 after controversial bankruptcy proceedings.
Staying in Russia, a foreign shareholder in OAO Gazprom, has filed a lawsuit against auditor PriceWaterhouseCoopers in the Russian courts.
The Guernesy-based Hermitage Fund, which specialises in the Russian and CIS equity markets has accused the auditor of producing "false and misleading audits" of the world's biggest gas company.
