In the statement dictated from his deathbed, Alexander Litvinenko accused the Russian leader of having "no respect for life, liberty or any civilized value." In his first public remarks on the allegations, Putin said he deplored the former spy's death but called the statement a political provocation.
The Health Protection Agency said the radioactive element polonium-210, which is extremely hard to detect, had been found in Litvinenko's urine.
Polonium-210 occurs naturally and is present in the environment at very low concentrations, but can represent a radiation hazard if ingested.
"Only a very, very small amount of polonium would need to be ingested to be fatal, but that depends on how pure the polonium is," said Dr. Mike Keir, a radiation protection adviser at the Royal Victoria Infirmary.
The agency's chief executive, Pat Troop, said that the high level indicated Litvinenko "would either have to have eaten it, inhaled it or taken it in through a wound."
"We know he had a major dose," she said.
Earlier, Home Secretary John Reid said Litvinenko's death Thursday night was "linked to the presence of a radioactive substance in his body."
Litvinenko, a vociferous critic of the Russian government, suffered heart failure late Thursday after days in intensive care at London's University College Hospital battling a poison that had attacked his bone marrow and destroyed his immune system.
"You may succeed in silencing me but that silence comes at a price. You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed," Litvinenko said in the statement read by his friend and spokesman Alex Goldfarb. The former spy said "the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life."
Goldfarb said Litvinenko had dictated the statement before he lost consciousness on Tuesday, and signed it in the presence of his wife, Marina.
Litvinenko told police that he believed he had been poisoned on Nov. 1. His hair fell out, his throat became swollen, and his immune and nervous systems were severely damaged.
CBS News Moscow Bureau Chief Beth Knobel reports that although he had been critical of Putin and his government, Litvineko was not widely known until he fell ill.
On the day he first felt sick, Litvinenko said he had two meetings, the first with an unnamed Russian and Andrei Lugovoy, a KGB colleague and bodyguard to former Russian Prime Minster Yegor Gaidar.
He also dined with Italian security expert Mario Scaramella to discuss the October murder of crusading journalist Anna Politkovskaya.
Scaramella said he showed Litvinenko an e-mail he received from a source naming Politkovskaya's killers, and naming other targets including Litvinenko and himself.
CBS News located Lugovoy, who said he didn't poison his friend.
"About a year ago, Alexander called me and suggested that we had a meeting next time I was in London," Lugovoy said. "I fly to London quite frequently. He told me he had some business proposals I could find interesting. He told me that he had contacts with a number of British companies who were interested in getting access to the Russian market and who would like to get some consulting services in this connection with an eye to finding partners in Russia."
Lugovoy said he was aware Litvinenko's reputation as a critic of the Russian government, and therefore, "I have always tried not to discuss anything but business with him. And we would usually meet for no longer than an hour, an hour-and-a-half each time I was in London. We would meet, we would go some place, discuss stuff and part."
He said the two met on Nov. 1 at a hotel bar, where they were joined by Lugovoy's business partner Dmitry Kovtun. Later, another business partner, Vyacheslav Sokolenko, as well as Lugovoy's wife and son joined the group. Lugovoy said Litvinenko did not eat anything, but that drinks were ordered.
Lugovoy adds, "Everyone says I met him before his meeting with the Italian. This is not right – I met him after his meeting with the Italian."