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.

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".

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".

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.