LONDON — A high-profile British inquiry into the poisoning of Alexander V. Litvinenko, a former K.G.B. officer turned critic of the Kremlin, concluded in a report released on Thursday that his murder “was probably approved†by President Vladimir V. Putin of Russia and the head of the country’s spy service. The finding by Robert Owen, a retired High Court judge, in a 328-page report, represented by far the most damning official link between Mr. Litvinenko’s death on Nov. 23, 2006, and the highest levels of the Kremlin.