Renaissance Capital, the Russian investment bank part-owned by billionaire Mikhail Prokhorov, said an equities trader lost $10 million on unauthorized bets amid the country's biggest market rout in a decade.

''The bank's risk controls quickly uncovered the incident, and we took immediate action to mitigate the risk to the firm,'' Renaissance spokesman Quinn Martin said by phone today.

The trader, Anton Stenin, no longer works for the company, Renaissance said. Stenin, 28, built up about $130 million in unauthorized positions using borrowed money in a client's account, the Vedomosti newspaper reported today.

The client, an industrial company, encouraged Stenin to violate risk rules starting in early September and then couldn't meet margin calls, according to a senior trader who declined to be identified because he's not authorized to speak to the press. Stenin walked out of the office before his trades were discovered and hasn't been located since, the trader said.

Stenin didn't respond to messages sent to his profile page on the social networking Web site Odnoklassniki.ru and attempts to locate a phone number for him were unsuccessful.

The RTS Index in Moscow tumbled 47 percent in the third quarter, making it the second-worst performer in the period of the 88 tracked by Bloomberg. The dollar-measured RTS has lost 65 percent this year.

Prokhorov, Jennings

Prokhorov and Renaissance Group founder Stephen Jennings announced Sept. 22 that Prokhorov's Onexim Group had agreed to buy 50 percent minus one share of Renaissance Capital for $500 million in cash. That was less than a quarter of the value the investment bank had a year earlier, when VTB Group sought to take it over, according to a Vedomosti report.

Onexim spokesman Igor Petrov declined to comment on the trading loss. Renaissance Capital had net income of $340 million in 2007, up from $301 million a year earlier. Renaissance Group includes asset management units and brokerages in Ukraine, Kazakhstan and sub-Saharan Africa.

In January, Societe Generale SA, France's second-biggest bank, blamed a trader for 50 billion euros ($68 billion) in unauthorized futures positions that cost it 4.9 billion euros to unravel.