A hospital spokesperson said that contrary to earlier reports, Litvinenko had not been poisoned with thallium, and radiation poisoning was also unlikely.
He said doctors were still uncertain as to the cause of his condition.
Sergei Ivanov, a spokesman for Russia's SVR foreign intelligence service, told Reuters: "We continue to deny our involvement in the incident which happened to the former employee of the FSB Alexander Litvinenko."
The SVR is the main successor to the KGB's first department, which organised Soviet intelligence operations overseas.
Britain has put its anti-terrorist police on the case, which could have diplomatic repercussions if high-level Russian involvement is shown. The Kremlin on Monday dismissed such talk as "pure nonsense".
"There is absolutely no interest for us in occupying ourselves with such activities. Of course everyone has worth, but this person is not worth enough ... to poison the atmosphere of warm relations between Moscow and London," Ivanov said.