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HONG KONG — Shares in BYD, a Chinese car company backed by Warren E. Buffett’s Berkshire Hathaway, plunged without explanation by as much as 47 percent on Thursday, losing more than $1 billion in market value for the day.

BYD, which is based in the southern city of Shenzhen and makes electric vehicles, batteries and mobile phone handsets, said it could not explain the sudden decrease, which coincided with surging trading volumes in its stock.

In an announcement to the Hong Kong stock exchange on Thursday afternoon, BYD said it was “not aware of any reasons for these price or volume movements or of any information which must be announced to avoid a false market in the company’s securities or of any inside information that needs to be disclosed.”

Shares in BYD rose in morning trading on Thursday, but plummeted in the afternoon session before recovering slightly and finishing the day down 29 percent. The company lost 9.3 billion Hong Kong dollars, or $1.2 billion, in market value.

Trading activity in the stock was extraordinarily high. Around 253 million shares in BYD changed hands on Thursday, nearly 20 times the average daily trading volume during the last month.

Investors and analysts were at a loss to explain the sell-off; Berkshire Hathaway has a 9 percent stake in the company, held through its MidAmerican Energy unit.

Local news reports in Hong Kong cited potential speculation that the company had come under financial stress because of the weakness of the Russian ruble. A day earlier, shares in Geely Automobile, another Chinese carmaker, had closed down 17 percent. Geely warned on Tuesday that its profit for 2014 was likely to fall around 50 percent, partly because of foreign exchange losses related to its operations in Russia.

But BYD has little to no exposure to the Russian market. Its largest market by far is mainland China, and the company also sells small volumes of electric cars and buses in Hong Kong, the United States and Europe, mostly for use on a trial basis by municipal taxi fleets or bus routes.