LONDON — Detectives are trying to piece together the recent movements of a former Russian security agent and critic of President Putin who is seriously ill in a hospital after being poisoned.

In an episode reminiscent of the Cold War, Alexander Litvinenko, 43, a former KGB colonel in exile in London since 2000, fell ill at the start of the month after meeting a contact in a London restaurant. He is said to have been investigating the recent murder of a Moscow journalist, Anna Politkovskaya.

Mr. Litvinenko started feeling sick after he was passed documents in a sushi restaurant, Itsu, in Piccadilly. His condition deteriorated last week and tests are said to have shown he had been poisoned with Thallium, an odorless, tasteless substance.

Scotland Yard launched an investigation on Friday and its officers have asked Itsu for any CCTV images.

Mr. Litvinenko and his friends suspect the Russian government and say he was probably poisoned during his restaurant rendezvous.

There was no suggestion yesterday that Mr. Litvinenko’s contact, an Italian academic, or anyone working at the restaurant was involved.

Reports so far have not suggested anything as dramatic as the murder in 1978 of the Bulgarian dissident Georgi Markov, who died after being shot in London with a ricin-tipped pellet from an umbrella gun.

However, the incident comes only weeks after the murder of Politkovskaya, a fierce critic of Russia’s role in Chechnya. Last week, before his condition worsened, Mr. Litvinenko said a contact had approached him to say they should talk and they arranged to meet at the restaurant. “He gave me some papers which contained some names, perhaps names of those who may have been involved in the murder of Anna Politkovskaya, and several hours after the meeting I started to feel sick,” he said.

He was said to be in a serious but stable condition in University College Hospital last night.

A clinical toxicologist, John Henry, who examined Mr. Litvinenko, said he believed he was given a potentially lethal dose of Thallium. “It is tasteless, colorless, odorless. It takes about a gram, a large pinch of salt like in your food, to kill you,” Mr. Henry said. “He is quite seriously sick. There’s no doubt that he’s been poisoned byThallium and it probably dates back to November 1, when he first started to get ill.”

One report suggested that Mr. Litvinenko suffered kidney and bone marrow damage and was vomiting regularly. Scotland Yard confirmed it was investigating the “suspicious poisoning,” but no one has been arrested. It declined to comment on suggestions Mr. Litvinenko was under armed guard.

Mr. Litvinenko is a contentious figure in Moscow, having alleged that members of the Federal Security Service, or FSB — the main successor to the Soviet KGB — had plotted to kill the tycoon Boris Berezovsky.

Mr. Berezovsky, who visited Mr. Litvinenko last week, said he suspected Russia’s intelligence services.

Mr. Litvinenko’s contact at the restaurant, Mario Scaramella, visited the British Embassy in Rome once he realized he was the last person to have seen Mr. Litvinenko.

He was interviewed by intelligence officials before leaving and, it is believed, going into hiding.

An Italian political source and close friend of Mr. Litvinenko said: “Mario is very scared. He is worried that the Russians and the Chechens are after him.”