The Russian former KGB colonel who was poisoned in Britain was moved into an intensive care unit yesterday as his condition worsened.

Pictures released from a hospital showed 43-year-old Alexander Litvinenko looking emaciated and with his hair falling out. His liver is beginning to fail under the effects of thallium, a toxic heavy metal that can kill even in small doses.

A few weeks ago, Mr. Litvinenko was fit and healthy and ran five miles a day as part of a rigorous fitness regime. Now he is receiving 24-hour specialist nursing care as he battles against the poison he unwittingly swallowed.

The head of Scotland Yard’s anti-terrorist department has taken over the investigation as police said they were anxious to trace two Russians whom Mr. Litvinenko met in a London hotel on the day he became ill.

Mr. Litvinenko’s friends and supporters pointed the finger of guilt at the Kremlin — an accusation that, if proved, would cause a major rift in relations between Britain and Russia.

Senior government officials in Moscow dismissed such claims as “sheer nonsense.” The Foreign Office refused to comment until Scotland Yard had completed its investigation.

One of the two Russians was named by Mr. Litvinenko’s friends as Andrei Lugovoy, a former KGB bodyguard and one-time head of security at a television station owned by the Russian billionaire Boris Berezovsky, a friend of Mr. Litvinenko’s.

Mr. Litvinenko, who is married with a son, is said to have known Mr. Lugovoy in Moscow. He did not know the other man at the hotel, whom he could only name as Vladimir.

There was no suggestion yesterday that either man was involved in the poisoning, but detectives want to eliminate them from their inquiries.

Mr. Lugovoy’s whereabouts were unclear last night, though it is believed he may be in Moscow. Mr. Berezovsky last night made a second visit to Mr. Litvinenko in the hospital.

“He is really in very bad shape, but personally he is strong,” he said. “He has the same opinion he had before he was poisoned. He personally thinks that it was organized in Moscow and Putin gave the order to poison him because he is former KGB. It is a message to everyone who is in the service that no one is able to leave without being killed or damaged very seriously.”

Mr. Litvinenko has lived with the risk of attack since he came to Britain, seeking asylum in 2000. In October 2004, petrol bombs exploded against the front of two houses in Muswell Hill, north London. Mr. Litvinenko, his wife, Marina, and their son were asleep in one of the houses. Two men were arrested. Sources said they were of Russian or Chechen nationality. However, it is understood they were released when the Litvinenkos declined to press charges.