Berezovsky, an outspoken critic of Vladimir Putin, the Russian president, had long feared for his life and had survived repeated assassination attempts — although none in recent years. He had been due to be a witness at the inquest next month into the death of Alexander Litvinenko, the former KGB spy who was murdered in 2006 after being poisoned by radioactive polonium-210 in what is believed to have been a Kremlin-sanctioned assassination.