The European Court of Human Rights found on Tuesday that Russia was responsible for the assassination of ex-KGB officer Alexander Litvinenko, who died an agonising death in 2006 after being poisoned in London with a rare radioactive substance.

Litvinenko, a defector who had become a vocal critic of the Kremlin, died three weeks after drinking green tea laced with polonium-210 at a plush London hotel.

Britain has long blamed the attack on...