By ALAN COWELL
LONDON — An inquiry into the poisoning death of Alexander V. Litvinenko, a former K.G.B. officer, whistle-blower and bitter foe of President Vladimir V. Putin of Russia, ran into a last-minute delay on Monday after one of the two suspects in the killing demanded a chance to clear his name and exonerate the Kremlin.
The development offered one more tangle in a saga distinguished by layers of claim and counterclaim, drawn in halftones from a murky world at the intersection of political dissent, clandestine intelligence-gathering and what British lawyers have depicted as intimate ties between the elite in Moscow and organized crime.
Mr. Litvinenko, 43, died in November 2006 after drinking tea that had been laced with a rare radioactive isotope, polonium 210. He had the tea during a meeting at a London hotel with two Russians — Andrei K. Lugovoi, a former K.G.B. bodyguard, and Dmitri V. Kovtun, a onetime Soviet Army officer. Both men have denied accusations by the British police that they killed Mr. Litvinenko, and both have remained outside Britain.
A high-profile public inquiry into the death began on Jan. 27, after years of resistance by the British and Russian authorities. It was scheduled to conclude this week, but Mr. Kovtun complicated matters several weeks ago when he declared that he wanted to testify and to become what is known as a core participant in the case. That status would grant him privileges including the opportunity to have his lawyers cross-examine witnesses.
The maneuver was widely seen by British lawyers as a ploy to derail the inquiry’s schedule. It introduced the first direct challenge to an account of the case that until then had been dominated by the British police and scientists, and that seemed to lead directly to the Kremlin.
On his deathbed in 2006, Mr. Litvinenko accused Mr. Putin of responsibility for his poisoning, a charge that the Russian leader has dismissed. Mr. Litvinenko’s death came six years after he fled Russia, and a few weeks after he and his family were granted British citizenship.
Mr. Kovtun’s request threw the final days of the inquiry into disarray.
At a hearing on Monday, Robert Owen, the senior judge in charge of the inquiry, set conditions for Mr. Kovtun to become a core participant including requirements that he produce written evidence and a witness statement by May 22 and that he answer a series of questions posed to him earlier.
The judge set July 27 as the date for Mr. Kovtun to begin testifying by video link. “I am ready to answer everything,” Mr. Kovtun told the BBC last week. “I had nothing to do with the murder.”
Speaking to reporters in Moscow, Mr. Kovtun gave the impression that he expected core participant status to offer him access to classified documents. But Judge Owen said that neither he nor the existing core participants, including the British police and Mr. Litvinenko’s widow, Marina, would be shown secret material. Nor would witnesses who have already testified be recalled, the judge said.
He added that Mr. Kovtun would be required to say by May 22 whether he planned to invoke a legal right to remain silent to avoid self-incrimination.
“He will be expected to cooperate fully with the inquiry,” Judge Owen said, adding that the July 27 date for testimony was “fixed, and it will not be moved.”
Mr. Kovtun’s demand to participate caught British lawyers between a profound skepticism about his motives and an equally powerful reluctance to be perceived as blocking testimony. Robin Tam, a counsel for the inquiry, said on Monday that Mr. Kovtun’s “intervention” should not be permitted to delay Judge Owen’s final report, expected this year.
Apart from public hearings, the judge also plans closed-door sessions with security officials and others whose contents will not be divulged, in compliance with a series of gag orders intended to safeguard what Judge Owen on Monday called “the national security or international relations.”
Before the inquiry started, the home secretary, Theresa May, insisted that it not cover the question of whether the British authorities ought to have protected Mr. Litvinenko. A lawyer for Ms. Litvinenko has said that her husband acted as a paid agent of MI6, Britain’s overseas intelligence agency, after he fled Moscow.
Throughout the inquiry, which has included 29 days of hearings so far, the Kremlin has remained aloof, declining to hand over either of the suspects.
British scientists have said the isotope that killed Mr. Litvinenko could only have come from Russia, which produces most of the world’s commercial supply of polonium, a rare metal once used in triggers of nuclear weapons.
“The murder was an act of unspeakable barbarism that inflicted on Alexander Litvinenko the most painful and lingering death imaginable,” Ben Emmerson, a lawyer for Ms. Litvinenko, said when the inquiry opened. “It was also an act of nuclear terrorism on the streets of a major city, which put the lives of numerous other members of the public at risk.”
He said Mr. Litvinenko was killed “not because he was an enemy of the Russian state itself or an enemy of the Russian people, but because he had become an enemy of the close-knit group of criminals who surround Vladimir Putin and keep his corrupt regime in power.”