PARIS — The highest court in France on Wednesday rejected a bid by George Soros, the billionaire investor, to overturn a conviction for insider trading in a case dating back nearly 20 years, leaving the first blemish on his five-decade investing career.

The panel, the Cour de Cassation, upheld the conviction of Soros, 75, an American citizen, for buying and selling Société Générale shares in 1988 after receiving information about a planned corporate raid on the bank.

Ron Soffer, his lawyer, said Soros planned to take the case to the European Court of Human Rights, saying that the length of the proceedings had prevented his client from having a fair trial.

"The investigation started in 1989," he said. "The appeals trial occurred in 2004. How can you call witnesses and ask them about what happened in 1988?" The French stock market regulatory authority investigated the matter separately and concluded that Soros had not violated the law or any ethical rules, Soffer said.