LONDON — Almost eight years after Alexander V. Litvinenko, a former K.G.B. officer turned whistle-blower, was poisoned in London with a radioactive isotope, the British authorities announced on Tuesday that a public inquiry would be held into his death, permitting investigators to explore whether Russian leaders had ordered the killing.

At a time when President Vladimir V. Putin faces mounting Western opprobrium for his support of pro-Russia separatists accused of shooting down a Malaysia Airlines jetliner over eastern Ukraine, the announcement from Theresa May, the British home secretary, could be seen by Moscow as a further rebuke.

“It is more than seven years since Mr. Litvinenko’s death, and I very much hope that this inquiry will be of some comfort to his widow,” Ms. May said in a written statement to Parliament.

The announcement reflected a remarkable about-face by the government of Prime Minister David Cameron, which, seeking improved relations with Moscow, long resisted demands by Mr. Litvinenko’s supporters for scrutiny of any role in his death by the Russian state or the British intelligence services. With Mr. Cameron calling for sanctions against Russia over the crisis in Ukraine, the ground now seems to have shifted, although British officials described the timing of Tuesday’s announcement as a coincidence.