Russia’s intelligence service said Friday that it had opened a criminal investigation into British espionage here in Russia, based on statements and undisclosed evidence provided by a businessman who is accused of poisoning Alexander V. Litvinenko, a former K.G.B. officer and a Kremlin critic.

The announcement by the Federal Security Service, the domestic successor of the Soviet K.G.B., further confused a murder investigation that has soured relations between Russia and Britain. The countries have staked out starkly different positions regarding the killing of Mr. Litvinenko and the motives behind it.

Mr. Litvinenko died in London in November after ingesting a radioactive isotope, polonium-210. Last month, British prosecutors accused a business associate of his, Andrei K. Lugovoi, of carrying out the poisoning, and demanded his extradition.

Mr. Lugovoi has denied the accusations. Two weeks ago, he held a theatrical news conference in which he accused Britain’s foreign intelligence service, MI6, of orchestrating the whole affair and recruiting Mr. Litvinenko and a prominent Russia tycoon in self-exile, Boris A. Berezovsky. He said they had conspired with British intelligence operatives to provide compromising information about President Vladimir V. Putin of Russia.

Mr. Lugovoi’s assertions were the basis for the new investigation, the Federal Security Service said Friday, though its one-sentence statement did not mention who was the focus of any potential espionage charges.

A spokesman, Sergei N. Ignatchenko, said in a telephone interview that the security service was investigating information that Mr. Lugovoi “did not voice at the press conference,” though at the time Mr. Lugovoi had suggested that he had incriminating evidence against Mr. Berezovsky and Mr. Litvinenko.

Mr. Ignatchenko added that the investigation was focused on “reconnaissance work of the British intelligence service on the territory of the Russian Federation.” His comments suggested that the investigation could also focus on Russians who may have cooperated with British operatives here.

Russia has accused Britain of spying before. In 2006, the Federal Security Service accused four British diplomats of communicating with Russian agents using a device disguised as a rock, though Russia chose not to expel them, as is standard practice in cases of suspected espionage.

Also last year, a Russian court convicted a retired officer, identified as Col. Sergei V. Skripal, of having passed classified information to MI6 for a decade. The court sentenced him to 13 years in prison.

The new investigation appeared to give an official endorsement to Mr. Lugovoi’s assertions. Although investigators here have said that they are looking into the circumstances of Mr. Litvinenko’s death, they have made it clear that Mr. Lugovoi is not a suspect. That has led to accusations by Mr. Litvinenko’s relatives and supporters that Russia was harboring an accused murderer.

Russia has refused to consider a British request for Mr. Lugovoi’s extradition to face charges, since the country’s Constitution forbids it. Officials and the news media here continue to be scornful of the accusations that Mr. Lugovoi might have been involved in the killing.

Russia’s prosecutor general, Yuri Y. Chaika, said Thursday that Britain had shared material that prompted the decision to accuse Mr. Lugovoi. But Mr. Chaika dismissed it as an “analysis of the evidence, no more than that.”

In London, a British official, who spoke on the condition of anonymity under civil service rules, said that prosecutors were “still awaiting the formal Russian response” to the extradition request. The official described the Litvinenko case as “a criminal matter, not an issue of intelligence.”

(Published by The New York Times, June 16, 2007)