Tim Summers, the most senior former BP manager at oil firm TNK-BP, said Tuesday that he would leave in mid-October for personal reasons. Another BP manager will replace him.

Summers was a key figure for BP in the fierce shareholder dispute that raged until last year between BP and the four Russian billionaires who co-own TNK-BP, but he said his departure was not related to that issue.

“After 3 1/2 years of TNK-BP, which has been pretty interesting and productive, it’s time to think about what to do next,” Summers said.

“I’m going to leave in mid-October. It’s a decision I’ve taken for personal reasons.”

TNK-BP will appoint the head of BP Azerbaijan, Bill Schrader, as its new chief operating officer, the company said in a statement.

TNK-BP accounts for about a quarter of BP’s worldwide output, but following the shareholder dispute BP ceded much of its influence at the company and a string of senior expatriate managers left.

Following raids on TNK-BP by the security service and a string of investigations by labor inspectors and migration authorities, the Russian billionaires were accused of using state law enforcement bodies to wrest management control.

They denied this, saying they were simply pursuing their legitimate rights as shareholders and attacking BP’s “arrogance” in allegedly treating the firm as one of its subsidiaries.

Investors were alarmed by talk that the Russian government was tacitly supporting the oligarchs against a major foreign investor, but officials always maintained that they had stayed out of what they termed a private dispute between shareholders.

Part of the resolution to the dispute was the agreed departure of TNK-BP’s then-CEO, a former BP executive named Robert Dudley, whom the Russians accused of favoring BP.

Dudley’s exit last December left Summers as the most senior former BP manager at the firm, which made net profits of $2 billion in the first half of this year.

Summers took over as acting CEO for six months before Russian oligarch Mikhail Fridman, one of the four co-owners, stepped in to replace him on an interim basis as CEO at the end of May. At that point, Summers returned to his COO role.

TNK-BP’s shareholders have still not agreed on a permanent chief executive to replace Dudley. Under the joint venture agreement, BP has the right to nominate the CEO, but the choice must be approved by the oligarchs, who are insisting on a fluent Russian speaker.