BY GREGORY L. WHITE
MOSCOW—Russian billionaire Mikhail Fridman resigned abruptly Monday as chief executive of TNK-BP Ltd., a sign of deepening tensions between BP PLC and its Russian partners at their 50-50 joint venture.
“The situation with corporate governance has become untenable,” said Stan Polovets, chief executive of AAR, the consortium through which the Russian shareholders, a group of Soviet-born billionaires led by Mr. Fridman, hold their TNK-BP stake.
BP admitted Mr. Fridman’s departure—more than a year before the end of his informal term, which was set to end in 2013 —was unexpected but played down the tensions. “His role as CEO has largely …