The US, UK and Germany have criticised the new six-year sentence imposed by a Russian court on former oil tycoon Mikhail Khodorkovsky.
Khodorkovsky, who is near the end of an eight-year term for tax evasion, has been told he will stay jailed till 2017 for embezzlement and money-laundering.
The US said the new sentence seemed to be an abuse of Russia's legal system.
Russia has not yet responded, but previously rejected Western criticism of the guilty verdict as interference.
After the sentencing, US state department spokesman Mark Toner said Washington was concerned by the apparent "abusive use of the legal system for improper ends, particularly now that Khodorkovsky and [former business partner Platon] Lebedev have been sentenced to the maximum penalty".
Later an unnamed senior US administration official, quoted by Reuters news agency, said the sentencing might complicate Russia's expected entry to the World Trade Organisation in 2011.
German Chancellor Angela Merkel said she was "disappointed" by the sentence.
"The impression remains that political motives played a role in the trial," she said in a statement.
And UK Foreign Secretary William Hague said he was deeply concerned and urged Russia "to respect the principles of justice and apply the rule of law in a non-discriminatory and proportional way".
"In the absence of this the UK and much of the international community will regard such a trial as a retrograde step," Mr Hague added.
Once seen as a threat to former President Vladimir Putin, he was found guilty along with Lebedev of stealing billions of dollars from their own oil firm, Yukos, and laundering the proceeds.
Their lawyers are expected to appeal but if Khodorkovsky does remain in jail until 2017, it will mean he does not return to society until well after the next Russian presidential election.
Some analysts have suggested he could otherwise pose a political threat to the Kremlin's candidate in 2012.
'Isolated from society'
Khodorkovsky and Lebedev were first arrested in 2003 and sentenced in 2005 for fraud and tax evasion.
On Thursday, the court in Moscow sentenced the two men to 14 years in prison, to run concurrently with the eight-year term handed down in 2005.
The term includes time served since the two men's arrest.
Judge Viktor Danilkin had been reading the 800-page verdict out since Monday.
Khodorkovsky could "only be reformed by being isolated from society", the judge said.
As sentence was passed, the defendant's mother shouted at the judge: "May you and your offspring be damned!"
The two defendants themselves, however, are said to have reacted calmly to the decision.
Supporters have held rallies outside the courthouse to condemn Mr Putin and the Kremlin.
Defence lawyer Yury Shmidt told reporters that the sentence amounted to "lawlessness".
He accused the Russian authorities "headed by Putin" of leaning on the justice system.
"Putin signalled to the court who today is the boss and who today decides Khodorkovsky's fate and life," he added.
Mr Putin referred to Khodorkovsky in a televised question-and-answer session last week, when he said he believed "a thief belongs in prison".
The defence has argued that the charges were absurd since the amount of oil said to have been embezzled would be equivalent to the entire production of Yukos in the period concerned.
After tax police filed enormous claims for unpaid taxes against Yukos, Khodorkovsky's old company filed for bankruptcy in 2006.