Platon Lebedev, the business partner and associate of jailed oil tycoon and outspoken Kremlin critic Mikhail Khodorkovsky, filed an appeal against his sentence on Thursday.

Khodorkovsky and Lebedev had been nearing the end of a separate eight-year sentence for fraud and tax evasion from their 2005 trial when they were told in December 2010 that they would have to stay in jail until 2017 during a trial widely condemned by the West. In June 2011, both men's sentences were reduced by one year.

"Today we filed an appeal against the rulings of a district court and the Moscow City Court... demanding to cancel the rulings of both courts as unlawful and groundless," Lebedev's lawyer, Konstantin Rivkin, said.

Lebedev is presently serving out his prison term in Murmansk and Khodorkovsky is in Russia's northwestern Karelia Region.

The lawyer of Khodorkovsky, Vadim Klyuvgant, filed a similar appeal on July 14.