Russian President Vladimir Putin said in an interview published today that he did not know jailed oil tycoon Mikhail Khodorkovsky had been put in a solitary punishment cell.
In an interview with Spanish journalists published on the Kremlin website, Mr Putin said he would talk to Justice Minister Yuri Chaika about Khodorkovsky's move to solitary confinement.
"Prison isn't a holiday camp," Mr Putin said in the interview ahead of a visit to Spain. "That Khodorkovsky found himself in a punishment cell, I can say openly, I heard first from you.
"As you have drawn my attention to this, I will ask the justice minister what is going on, where they have sent him and for what."
Mr Khodorkovsky clashed publicly with Mr Putin before his arrest about high-level corruption, oil pipelines and the distribution of oil wealth.
His press service said he was put in a punishment cell for five days last month at the Siberian prison where he is serving an eight-year sentence for fraud and tax evasion.
He was put in the cell, which Russian media reported was poorly heated in the depths of Siberia's bitter winter, for having documents that were banned in prison.
His lawyers said at the time the documents had been checked by guards, and they
accused the prison authorities of discriminating against Khodorkovsky.
Once Russia's richest man, Khodorkovsky made a fortune in the 1990s when he gained control of one of the gems of the country's oil industry.
He was convicted in May and his appeal was thrown out in September. He says the charges were part of a plan by Kremlin officials to take control of the oil corporation Yukos, which he once controlled.