Russia's foreign intelligence service has denied poisoning a former spy turned Kremlin critic in London, the Interfax news agency has reported.
Alexander Litvinenko is being treated in hospital
Alexander Litvinenko is now seriously ill and had recently received e-mail death threats, a friend has claimed.
SVR spokesman Sergei Ivanov is reported as saying it would not be in Russia's interests to "engage in such activity".
Meanwhile, doubts persist over whether the substance used was the toxic metal thallium, or something else.
Mr Ivanov is quoted as saying: "Mr Litvinenko is not the kind of person for whose sake we would spoil bilateral relations.
"It is absolutely not in our interests to be engaged in such activity."
The SVR is one of the successor agencies to the KGB, which became world renowned during the days of concerted international espionage in the Cold War.
Toxicologist Professor John Henry said that the poison may have been a radioactive form of thallium, which would now be difficult to trace, but that some other substance was also involved.
The doctor treating the 43-year old said in the hospital's first official statement that the cause may never be found.
Dr Amit Nathwani said it was possible he may not have been poisoned by thallium.
Dr Henry said Mr Litvinenko's bone marrow was not functioning and his white cell count has dropped to zero.
"Something other than thallium is involved," he said.
Radioactive thallium is used in hospitals, but Dr Henry said it was not used in massive doses consistent with Mr Litvinenko's condition.
Dr Henry added: "In this case his symptoms are gastro-intestinal so the probability is that he has swallowed something that is poisoned.
"Radioactive thallium degrades very rapidly so that by now we've missed the chance."
On Tuesday, Italian Mario Scaramella told a Rome press conference that he met Mr Litvinenko the day he fell ill and that both of them had received death threats.
Scotland Yard anti-terror officers have taken charge of the investigation.
Officers are treating the case as a suspected "deliberate poisoning".
Police have not yet identified the "time window" in which the poison was administered but believe it was in the last two weeks.
Mr Litvinenko is described as seriously ill but stable and has been moved into intensive care as a precautionary measure.
BBC Rome correspondent Christian Fraser said Mr Scaramella, who is involved in an Italian parliamentary inquiry into KGB activity, was sufficiently worried by the contents of an e-mail to ask for advice from Mr Litvinenko.
He said he met the Russian in a London sushi bar on 1 November for 35 minutes to discuss the e-mail.
Journalist Anna Politkovskaya was gunned down in Moscow last month
He said Mr Litvinenko had promised to look into the message but when Mr Scaramella called him later that night the Russian was already falling ill.
Meanwhile, detectives are planning to travel to Italy to interview Mr Scaramella and are also investigating whether Mr Litvinenko was being followed.
Mr Litvinenko had been investigating the murder of Russian journalist Anna Politkovskaya, an outspoken critic of Mr Putin and Russian policy in Chechnya, who was shot dead at her Moscow apartment building last month.