Professor John Henry said Alexander Litvinenko, 41, had symptoms consistent with thallium poisoning but other symptoms linked to other substances.
"It's not 100% thallium," Dr Henry said outside University College Hospital.
He said the poison may have been radioactive thallium, which would now be difficult to trace.
He said: "It may be too late. If it's a radioactive poison with a short half-life it may have gone.
"Radioactive thallium degrades very rapidly so that by now we've missed the chance [to trace the poison]."
Radioactive thallium is used in hospitals but Dr Henry said it was not used in massive doses consistent with Mr Litvinenko's condition.
The critic of Russian President Vladimir Putin fell ill on 1 November after a meeting at a London sushi bar.
Scotland Yard anti-terrorist officers have taken charge of the investigation into the poisoning by thallium of a former KGB colonel living in the UK.
The hospital said his condition was unchanged overnight.
The Kremlin has dismissed as "sheer nonsense" claims it was involved.
Friends of Mr Litvinenko have alleged he was poisoned because he was critical of the Russian government.
Kremlin spokesman Dmitry Peskov said: "We cannot comment on the very fact of what happened to Litvinenko.
"We don't consider it possible to comment on the statements accusing the Kremlin because it is nothing but sheer nonsense."
Russia's foreign intelligence service has issued a statement denying any involvement.
In a statement on Tuesday, University College Hospital said: "Mr Litvinenko's condition remains unchanged from yesterday. He remains in a serious condition in intensive care."
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.
Mr Litvinenko fled to the UK in 2000 and was granted asylum
Doctors had told Mr Litvinenko he had a 50/50 chance of surviving the next three to four weeks, said Alex Goldfarb, who has been visiting him.
Scotland Yard said it was treating Mr Litvinenko's illness as a suspected "deliberate poisoning".
Officers are interviewing possible witnesses, including Mr Litvinenko, and examining CCTV footage, it said in a statement.
"We await the results of the toxicology tests and we are therefore not speculating as to the possible cause of his condition at this stage," the statement added.Güncelleme Tarihi: 20 Eylül 2018, 18:16