Judicial authorities said Thursday that they had taken the unusual step of appointing a senior judge to hold a long-planned inquest into the death of Alexander V. Litvinenko, a former K.G.B. officer poisoned in London in 2006 with a rare radioactive isotope, polonium 210. The case soured relations between Britain and Russia, and the new development may put at risk recent improvements. At British inquests, judges replace coroners only in the most high-profile cases, like the inquiry into the death of Princess Diana. The hearings to be overseen by the judge, Sir Robert Owen, could provide the first opportunity in the case for witnesses to testify under oath. A procedural hearing is to be held next month. British prosecutors have accused another former K.G.B. officer, Andrei K. Lugovoi, of Mr. Litvinenko’s poisoning — a charge he denies — and are seeking his extradition, which Russia has refused. Mr. Litvinenko, who fled Russia in 2000, had secured British citizenship shortly before he died, and he accused Russia’s leader, Vladimir V. Putin, of responsibility for his poisoning.