MOSCOW (Reuters) - A Russian regional court on Friday struck down a ruling that cut the prison term for former oil tycoon Mikhail Khodorkovsky's business partner, denying him a rare legal victory in a case criticized as politically motivated.

Khodokovsky and Platon Lebedev were arrested in 2003 in what they say was a Kremlin-driven campaign to increase state control over the lucrative oil industry and punish ex-Yukos head Khodorkovsky for challenging the political domination of President Vladimir Putin.

The fate of Khodorkovsky, whom Putin has sharply criticized in public comments over the years, is being closely watched for signs of any softening of the president's position during a new six-year term he started on May 7.

Khodorkovsky, once Russia's richest man, and Lebedev, are serving 13-year prison terms on convictions for fraud, tax evasion, theft and money laundering.

A district court in the town of Velsk, where Lebedev is serving his term, cut the 55-year old's sentence by more than three years in August, arguing legal changes had softened punishments for financial crimes including money laundering.

A spokeswoman for the Arkhangelsk regional court, which acts as a court of higher instance over Velsk, said however that the August ruling was determined to have had legal faults.

"The (Arkhangelsk) court overturned the decision by the Velsk court and sent it back to the same court for reconsideration, though the judges (handling the case) will be different," said Ksenia Soloveva.

The motion of the Velsk court to trim Lebedev's term would have paved the way for his release in March 2013. Prosecutors argued his sentence should only be shortened by less than two years.

In trials that damaged Russia's image in the West during Putin's tenure in power, Khodorkovsky and Lebedev were convicted of fraud and tax evasion in 2005 and found guilty of theft and money laundering at a second trial in 2010.

"Everything is back to where we started. We can assume the previous ruling never happened," said Alexei Miroshnichenko, a lawyer of Lebedev, said of the latest court ruling.

Recent changes to Russian legislation indicate that Putin, who is facing the largest protests against his 12-year rule including four as prime minister, will remain loath to tolerate dissent in his new term.

(Reporting by Maria Tsvetkova, Writing by Gabriela Baczynska, Editing by Thomas Grove)