British doctors treating a gravely ill former Russian spy in a suspected poisoning case say his condition has worsened but they ruled out thallium or radiation as the cause.

A critical care specialist, Doctor Geoff Bellingan, told reporters in London that patient Alexander Litvinenko is now in critical condition. He said doctors now are convinced that the patient's condition is not the result of poisoning by the heavy metal thallium or radioactive substances. He said they remain at a loss to explain the illness.  

A friend of Litvinenko earlier said the 53-year-old patient had suffered heart failure and is now on a ventilator.

British anti-terrorism police are investigating the poisoning as an apparent murder attempt.

Litvinenko's friends accuse Russian operatives of trying to kill him for his outspoken criticism of President Vladimir Putin.  Russia's Foreign Intelligence Service has strongly denied involvement.

Litvinenko says he fell ill November first, after eating lunch at a sushi bar with a former intelligence operative.  The operative, Italian academic Mario Scarmella, emerged from hiding on Tuesday, telling reporters in Rome that he met with Litvinenko to confer about a hit list received from an unidentified source.

Scarmella said  both his and Litvinenko's name were on the list.  He further alleged that those who drew up the list were also responsible for the unsolved October murder of noted Russian journalist Anna Politkovskaya in Moscow.

Some information for this report was provided by AFP, AP and Reuters.