Moscow, Russia (CNN) -- A judge in Moscow sentenced Mikhail Khodorkovsky, the former head of Russia's Yukos oil company, and his business partner to 14 years in prison on corruption charges Thursday.
The sentence effectively adds six years behind bars for Khodorkovsky, since the judge set the sentence to begin in 2003 when he was initially imprisoned on other charges. Instead of a release in 2011, he will now be freed in 2017.
Khodorkovsky and partner Platon Lebedev were accused of stealing billions of dollars' worth of oil from Yukos production subsidiaries from 1998 to 2003. Khodorkovsky has already been convicted of underpaying taxes on the oil and is serving an eight-year sentence for fraud and tax evasion.
"The verdict is illegal, we will of course appeal it," said Khodorkovsky's lawyer, Yuri Schmidt. "It was delivered under pressure from the executive branch which is still headed by Mr. Putin." He added the verdict came under pressure from "people who are interested in keeping Mr. Khodorkovsky in prison."
Russia was hit with international criticism after the former oil tycoon was convicted of money laundering and embezzlement earlier this month. The United States, in particular, called it a case of selective prosecution.
U.S. State Department spokesman Mark Toner said after Thursday's sentencing that Washington remains "concerned by the allegations of serious due process violations, and what appears to be an abusive use of the legal system for improper ends, particularly now that Khodorkovsky and Lebedev have been sentenced to the maximum penalty."
"Simply put, the Russian government cannot nurture a modern economy without also developing an independent judiciary that serves as an instrument for furthering economic growth, ensuring equal treatment under the law, and advancing justice in a predictable and fair way," Toner added.
Khodorkovsky had expressed a desire to run for office at one time and funded opposition political parties. He has said the trial was part of a Kremlin campaign to destroy him and take the company he built from privatization deals of the 1990s.
The Kremlin has denied any role.
"Allegations about some kind of selective prosecution in Russia are groundless," the Russian Foreign Ministry said in response to the criticism. "Russian courts deal with thousands of cases where entrepreneurs are prosecuted."
Khodorkovsky's son, Pavel, told CNN after the conviction that his father will appeal the verdict.
Pavel Khodorkovsky said that he had been expecting a guilty verdict for his father, but also "had hoped that President (Dmitry) Medvedev's rhetoric about judicial system reform would actually bear some fruit."
"However, today, I realize that the judge is completely subservient and is a slave to the political will of Mr. (Russian Prime Minister Vladimir) Putin and other bureaucrats in the Kremlin," the younger Khodorkovsky said.
The Yukos oil company was once Russia's largest oil producer. It was later broken up and absorbed by the state.
Khodorkovsky previously told the Moscow court that its decision will have an impact far beyond him and Lebedev.
"There is much more than just the fates of two people in your hands," Khodorkovsky said. "Right here and right now, the fate of every citizen of our country is being decided."
In October, prosecutors asked for a 14-year prison sentence but said it should include the eight-year term that Khodorkovsky is already serving, which will end in October 2011.
The former oil magnate was previously incarcerated in a work camp near the town of Krasnokamensk, 6,500 kilometers (4,000 miles) from his native Moscow, but when the new charges were brought against the two men, both were moved to Moscow last year to stand trial.
The court also ordered Khodorkovsky and Lebedev to pay about $600 million in back taxes.
Few doubt that Khodorkovsky's prosecution has taken on a symbolism far beyond his own innocence or guilt. Critics say the embezzlement charges against him are absurd; analysts say Russia itself, and its commitment to the rule of law, is on trial.
"This verdict will be a verdict on whether Russia is a law-governed state, or whether it ever aspires to become one," said Masha Lipman of the Carnegie Endowment, "because currently it is not a law-governed state and the trial of Khodorkovsky and his partner Lebedev is the most striking example of it."
In his final address at his trial, Khodorkovsky made a last impassioned appeal.
"For me, as for anybody, it is hard to live in jail, and I do not want to die there. But if I have to, I will not hesitate. The things I believe in are worth dying for," he said.
CNN's Maxim Tkachenko contributed to this report.