Andrei Lugovoi, a businessman and former security service officer, earlier said he was willing to meet with Scotland Yard detectives, currently in Moscow investigating the murder of his fellow security officer Alexander Litvinenko.
"An interview has been scheduled for today, but Lugovoi is still undergoing medical checks, and he has not been questioned so far," the source said, adding that the questioning could be postponed until Thursday.
U.S. TV channel ABC News said Wednesday, citing an unidentified British official, that investigators were treating Lugovoi as the main suspect in the November 23 death by radiation poisoning of Litvinenko, who defected from Russia in 2000 and was known as an outspoken Kremlin opponent.
Lugovoi told The Times and The Daily Telegraph last week that he and two of his business partners had met with Litvinenko in London on November 1, the day the former security officer, a British national, is believed to have received a fatal dose of radioactive polonium-210.
Traces of radiation have been detected in the hotel rooms in London where Lugovoi stayed in October and November, and on the airliners in which he flew to the UK, the popular Russian daily Kommersant said Wednesday.
ABC also claimed Russian authorities had hindered British investigators' efforts in Moscow.
Russia's top prosecutor said Tuesday Lugovoi might be questioned if doctors gave their permission. But Yury Chaika said Scotland Yard detectives would not be able to arrest and extradite Russian suspects that may emerge in the case as this would contradict the Russian Constitution.
He also said any interviews of suspects in Scotland Yard's investigation would be carried out by Russian prosecutors.
