Khodorkovsky — the former head of Yukos oil company, and once Russia's richest man — was sentenced to prison for eight years in 2005 on charges of tax evasion and fraud in what Kremlin critics slammed as a flawed trial.

"Prisoner Khodorkovsky does not deserve conditional early release," Judge Igor Faliliyev said at the Ingodinsky regional court in the Siberian city of Chita, citing as reasons his refusal to take part in professional training in sewing while in prison, and an alleged misdemeanor dating back to October.

Khodorkovsky showed little surprise at the decision, appearing calm as the judge finished speaking. As he was hustled out of the courtroom by guards, he said Russia's "legal system will not be reformed anytime soon."

Detained since 2003, Khodorkovsky has served more than half of his sentence and has been eligible for parole for the past 10 months.

Most of that sentence has been served in the remote Chita region, nearly 4,000 miles east of Moscow. In December 2006, he was moved from a prison camp to a pretrial detention center in the city after new charges were brought against him and his business associate Platon Lebedev.