Litvinenko, 43, remains in serious condition and under police guard while experts try to unravel the mystery of what type of poisoning caused him to fall so ill and police detectives try to uncover who or what was behind it.
"We have requested toxicology tests to establish what poisoned Mr Litvinenko," University College Hospital in central London said in a statement.
"Based on results we have received today and Mr Litvinenko's clinical features, thallium poisoning is an unlikely cause of his current condition."
Litvinenko is in intensive care "so he can receive cardiac monitoring and specialist support in areas such as nutrition and pain relief.
"He can also be more effectively isolated to protect him against infection, following the damage to his immune system."
Doctor Amit Nathwani, the consultant caring for Litvinenko, told reporters outside the hospital that the former spy was suffering from a disturbance in the rhythm of his heartbeat.
"It is possible that he may not have been poisoned with thallium, although we cannot completely exclude this because of the timing of his presentation at our hospital," he said.
"We are exploring other possibilities and other avenues, but it is also quite possible that we may never find the ultimate cause."

Copyright AFP (Agence France-Presse), 2006