The mystery surrounding the poisoning of the former spy Alexander Litvinenko deepened today after doctors treating him in hospital said the cause of his sickness was unlikely to be from the toxic metal thallium.

Mr Litvinenko, who was a lieutenant-colonel in the FSB, the successor to the KGB, is believed to have been poisoned on November 1. He was initially admitted to hospital suffering symptoms of food poisoning but as his condition deteriorated toxicology tests were carried out which suggested that he had been poisoned by thallium, a soft, malleable metal, and another chemical toxin.

This morning a leading toxicologist said that Mr Litvinenko could have been poisoned by radioactive thallium, which would make his chances of survival even slimmer. However, further tests at University College Hospital today have revealed that the poison was unlikely to be thallium.

Dr Amit Nathwani, the consultant caring for Mr Litvinenko said: "We have done a series of investigations here and based on his presentation and some of the laboratory tests that have come through, 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.

"We are exploring other possibilities and other avenues but it is also quite possible that we may never find the ultimate cause."

Dr Nathwani said hospital staff were carrying out a further tests and investigating "numerous" other potential causes of his illness, that could explain his combination of symptoms. However, Dr Nathwani added of Mr Litvinenko: "He has been poisoned so we are looking for other causes of poison.

"His symptoms are slightly odd for thallium poisoning and the levels of thallium we were able to detect are not the kind of levels you would see in toxicity."

Dr Nathwani added that Mr Litvinenko had been transferred to an intensive care unit for the precautionary monitoring of his heart and that he had already had some treatment but it was a question of how he responded, adding: "We are talking about weeks and months."

Earlier today, the leading toxicologist Professor John Henry said the damage to Mr Litvinenko’s bone marrow and his blood cells suggested a large dose of "radioactive thallium" could be to blame for his condition. Dr Nathwani said tonight that radioactivity remained a consideration but that, at present, this was "speculation".

University College Hospital later clarified in a statement that it was unlikely Mr Litvinenko’s illness was caused by thallium poisoning. "We cannot rule out the possibility that Mr Litvinenko’s condition was caused by a radioactive material - including radioactive thallium - although not all of his signs and symptoms are consistent with radiation toxicity.

"Further tests will be carried out to establish whether or not there is a single cause for Mr Litvinenko’s condition," it said.

Counter-terrorism officers from Scotland Yard took charge last night of the investigation into the suspected deliberate poisoning of Mr Litvinenko, who fled Moscow in 2000 for London, as the first pictures were published his dramatically changed appearance.

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