Barnet Hospital has unwittingly found itself involved in the mysterious suspected poisoning of former Russian spy Alexander Litvinenko.

It emerged he was treated at the hospital for two weeks as his condition deteriorated.

Mr Litvinenko, 43, who lives in Muswell Hill, was yesterday in a serious but stable condition in intensive care at University College London Hospital (UCLH), after he was transferred there from Barnet Hospital on Friday.

He was admitted to Barnet on November 3 with what doctors at first believed was simple case of gastroenteritis - inflammation of the stomach and intestines. But tests revealed this not to be the case.

Dr Andres Virchis, the consultant haematologist at Barnet Hospital, who treated Mr Litvinenko, said it was not until four days after he was admitted that he revealed he was a former spy for the Russian Federal Security Services (FSB) - the successor to the infamous KGB. Upon admission, Mr Litvinenko used his British name, a pseudonym he assumed after defecting six years ago.

Dr Virchis said: "He was first presented at the hospital with a two-day history of vomiting and diarrhoea. He was actually about to be discharged on November 7 because he was suffering just diarroea and vomiting. Then we got the result of a test which revealed a bug that needed treatment. That was the first time he said who he was and wondered if he could have been infected with this on purpose.

"He mentioned that he heard of the bug before and of potential other cases of people being purposely infected and wondered if that had happened to him. Knowing who he was, there was some concern, but it was just a concern. It wasn't until about November 10 or 11 that it became clear this was not just gastroenteritis: he was becoming more and more ill, his hair began to fall out and his blood-counts were beginning to drop. That was when it became very clear there was more to this than meets the eye.

"He became increasingly - and his family became increasingly - concerned that there was something seriously amiss here."

Tests sent to Guy's Hospital, in central London, confirmed those fears, showing significantly excess levels of the heavy metal thallium. Last Friday, with the results in and Mr Litvinenko health rapidly deteriorating, he was transferred to UCLH.

On Tuesday, a UCLH state-ment said: "Based on results we have received today and Mr Litvinenko's clinical features,thallium sulphate poisoning is an unlikely cause of his current condition.

"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."

People close to Mr Litvinenko, a critic of the Russian Government, have accused the Kremlin of being involved. Mr Litvinenko is said to have been investigating the murder of Russian journalist Anna Politkovskaya, herself an outspoken critic of the Kremlin.

Scotland Yard's new Counter Terrorism Command, an amalgamation of Special Branch and Anti-Terrorist Branch, is investigating Mr Litvinenko's poisoning.