Russia's most famous prisoner of conscience has spoken out from behind bars to urge David Cameron to confront the Kremlin on human rights issues when the Prime Minister visits Russia next week.
Mikhail Khodorkovsy, the former head of the Yukos oil company.
In written comments passed to The Daily Telegraph from his prison in northern Russia, Mikhail Khodorkovsky said Mr Cameron should use the fact that top Russian politicians bank, own property, and educate their children in the UK to pressure the Kremlin into becoming "a modern European state."
"I would hope that the British Prime Minister will directly raise questions of corruption and the judicial system in Russia with President (Dmitry) Medvedev," the 48-year-old oligarch-turned-political prisoner wrote.
"The UK could let Russia understand that a country with such widespread corruption, the only G8 country where there are political prisoners, cannot be a fully-fledged and all-round partner."
His mother, Marina Khodorkovskaya, went even further. In an interview, she urged Mr Cameron to deny entry to Russian officials involved in her son's case and to freeze their UK bank accounts.
Mr Cameron is due to make a flying visit to Moscow on Monday during which he is expected to meet Mr Medvedev and Vladimir Putin, the prime minister.
He will be the first British leader to visit Russia since Tony Blair five years ago, and the first since the 2006 poisoning of Alexander Litvinenko, a former Russian spy and Kremlin critic, in central London.
The Prime Minister is aware of Mr Khodorkovsky's fate which has become emblematic of Russia's heavily politicised and selective justice system.
Designated as a prisoner of conscience by Amnesty International, the former oil tycoon was Russia's richest man before he had a spectacular falling out with Mr Putin in 2003 over business and politics.
Arrested by masked men on an icy Siberian runway in his private jet that same year, he was later found guilty of fraud and given an eight year jail term after a trial that was widely viewed in the West as politically-motivated.
He spent much of that time in a grim Soviet-style labour camp 3,000 miles east of Moscow near China.
Yukos, his oil company, was swallowed up by the Kremlin while he languished behind bars, and in a second trial last year he was sentenced to a further six years in jail after being found guilty of the surreal charge of stealing oil from own company.
In his written comments from his jail in northern Russia, Mr Khodorkovsky said he wanted his own case to be a warning to British companies considering investing in Russia.
"It will be difficult for anyone to expect British companies to do serious business in Russia whilst entrepreneurs are factually subjected to a bureaucratic racket," he wrote.
Although Russia and the UK do in fact enjoy strong business links, political relations have stalled over differences on key foreign policy, security, business and rights issues.
Companies in the West are keen to tap into Russia's status as the world's biggest energy exporter but Russia's liberal opposition argues that Western politicians such as Mr Cameron need to keep reminding the Kremlin it must enact real democratic reforms if it wants large-scale investment.
Mr Khodorkovsky said he thought the UK could start by withholding the transfer of technology necessary for Russia's modernisation until the Kremlin changed course politically. He claimed that the UK had real clout with Russia.
"There is no question that such leverage does exist," he wrote. "The most influential people in Russia, those who in large measure determine the image and the fate of the country today, have vast business and personal interests in the UK. This applies also to a series of significant representatives of Vladimir Putin's team."
Speaking at a school outside Moscow that Mr Khodorkovsky originally set up for orphans, his mother Marina, 77, said she wanted Mr Cameron to raise her son's case with the Kremlin.
But she said she was looking for more that just rhetoric. "Words need to be backed with action," she said, urging the UK to impose visa and financial sanctions on Russian officials such as judges and prosecutors involved in her son's case.
"It is realistic and it would have an effect," she argued. "If the people in the second echelon of power beneath Putin understand that the higher-ups cannot protect them they will soon forget their loyalty to the system."
Clear-eyed despite her advanced years, she said that she feared that her son would not be released for as long as Mr Putin wielded influence in Russia. The Russian prime minister bore her son a personal grudge for publicly cutting across his political and business interests, she insisted.
"My aim is to live long enough to see my son freed," she said as tears formed in her eyes. "But I understand that the chances are getting less and less as I get older and older."