The 2003 arrest of the oil billionaire generated interational controversy.

Vladimir Putin, who was Russian president at the time, was criticised by Western business figures who said Khodorkovsky was targeted because of his political ambitions.

Khodorkovsky has always maintained his innocence, saying he was the victim of corrupt officials under Putin who wanted to carve up his business empire.

Following Khodorkovsky's 2003 arrest, Yukos - one of the country's largest private firms- was broken up and its main assets sold off to a state-controlled company.