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.