"We have not faced any difficulties so far," Yuri Chaika told RIA Novosti in an interview.
Litvinenko died November 23 of last year after being poisoned with the radioactive substance Polonium-210. In December, a Scotland Yard team arrived in Moscow to question two key suspects in the case, spies-turned-businessmen Andrei Lugovoy and Dmitri Kovtun.
Earlier this year, Scotland Yard handed the Litvinenko case over to the Crown Prosecution Service, and the Russian Prosecutor's Office launched an inquiry of its own.
Chaika said the Russian investigators had already met for consultations with officials from the Crown Prosecution Service, the Home Office, and Scotland Yard. But he stopped short of giving a timeframe for their stay in the U.K.
"This is up to the British side to decide," he said.
Exiled Russian tycoon Boris Berezovsky told business daily Vedomosti that the Russian investigators will question him and fellow exile Ahmed Zakayev later this week in connection with the Litvinenko case.
Berezovsky and Chechen rebel emissary Zakayev were both granted political asylum in the U.K. after fleeing Russia, where they are wanted on charges of fraud and complicity in terrorism, respectively.
The Russian Prosecutor's Office has repeatedly approached British authorities with a request that the two men be extradited to their country of origin, but has been turned down every time.
Berezovsky told Vedomosti that the investigative team had initially planned to question more than 100 witnesses, but that only he and Zakayev gave their consent.
Other people from Litvinenko's inner circle, including his widow, Marina, his friend Alexander Goldfarb and several former executives of the oil giant Yukos, refused to cooperate with the prosecution, he said.
The oligarch said he had agreed to be questioned by the Russian prosecution team on condition that the interview not take place on the premises of the Russian diplomatic mission, and that it should be done in the presence of British police personnel and be videotaped.
Litvinenko was charged with abuse of office in Russia in the late 1990s after he publicly claimed he had been ordered by his superiors at the Federal Security Service (FSB) to assassinate Berezovsky.
He met the oligarch through Goldfarb on his arrival in the U.K. several years later, and began working for him as an adviser.
Goldfarb, the man who had arranged for the former spy to defect to the West, is the executive director of the International Foundation for Civil Liberties, a New York-based rights organization set up by Berezovsky in 2000.
