"You have shown yourself to have no respect for life, liberty or any civilized value. You have shown yourself to be unworthy of your office, to be unworthy of the trust of civilized men and women," Litvinenko's statement said.

"You may succeed in silencing one man, but the howl of protest from around the world, Mr. Putin, will reverberate in your ears for the rest of your life. May God forgive you for what you have done."

It's not clear when Litvinenko wrote the statement.

A Kremlin spokesperson said Friday the death of a person is always a tragedy and that the investigation is in the hands of police.

Litvinenko died at 9:21 p.m. local time Thursday at London's University College Hospital. A spokesperson for the hospital said New Scotland Yard would be investigating the death.

Friends said Litvinenko had been on a quest to uncover corruption in Russia's Federal Security Service, or FSB, and unmask the killers of another trenchant critic of the Putin government, investigative journalist Anna Politkovskaya.

The 43-year-old's condition deteriorated rapidly Thursday as heart failure left him on life support.

A fierce critic of the Russian government, Litvinenko had been under heavy sedation as doctors struggled to find the cause of his illness.

Early reports suggested he was the victim of thallium or radiation poisioning, but those theories were later ruled out.

His friend, Andrei Nekrasov, told the Associated Press that he spoke to Litvinenko before he lost consciousness on Tuesday, telling him: "The bastards got me, but they won't get everybody."

Kremlin spokesman Dmitry Peskov on Monday called the poisoning allegations "sheer nonsense."

Litvinenko said he fell ill after eating a sushi dinner on Nov. 1 with a contact who reportedly had information connected to his investigation of Politkovskaya's murder. She was shot in her Moscow apartment building in October.

His hair fell out, his throat became swollen and his immune and nervous systems were severely damaged.

Litvinenko's wife, father and one of his three children were with him when he died.

His father, Walter Litvinenko, who wept as he spoke outside the hospital, said his son was killed "by a little, tiny nuclear bomb.

"This regime is a murderous danger to the world ... If we just let it go, if we go about our daily business as usual, this regime will get at all of us."

A former spy for the KGB and FSB, Litvinenko fled to Britain in 2000 after spending nine months in a Russian jail. He had been charged with abuse of office, but was later aquitted.

In 1998, Litvinenko went public with allegations he was ordered to kill Russian tycoon Boris Berezovsky, who is exiled in Britain.

Litvinenko recently received British citizenship.

HPA teams were at the two London hospitals where Mr Litvinenko was treated to assess whether staff or family who had come into contact with him were now at risk, Ms Troop said.

"Our role is to assess the risk to the public and those who might have been in contact with this man," Ms Troop said.

She added that HPA staff had worked throughout the night after the agency was alerted to the presence of radiation at 6pm yesterday.

Roger Cox, director of the HPA's centre for radiation, chemical and environmental hazards, said small doses of polonium 210 could lead to increased cancer risks later in life, but only if it was ingested by people who came into contact with a contaminated substance. At high doses there would be enough damage to bone marrow, intestines and other organs to cause them to malfunction.

"With regards to this incident at 6pm yesterday, the HPA received information via the police that a large quantity of alpha radiation, probably from a substance called polonium 210, had been detected in the urine of Mr Litvinenko," he said. "The links between his symptoms and this exposure is a matter for the police."

He said polonium 210 occurred naturally and was present in some foods in low doses. It was a "pure alpha emitter", which meant its short radiation particles would not pass though solid substances, such as skin, but could be ingested.

Earlier, Mr Litvinenko's friend Alexander Goldfarb read out a statement dictated by the 43-year-old in the presence of his wife, Marina, on Tuesday, two days before he died.

Doctors said they believed Mr Litvinenko had been poisoned, but they did not know what with.

"You may succeed in silencing me, but that silence comes at a price," the statement said. "You have shown yourself to be as barbaric and ruthless as your most hostile critics claim.

"You may succeed in silencing one man. But a howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life. May God forgive you for what you have done."

Mr Litvinenko, who has been living in London since defecting from Russia in 2000, said: "I am honoured to be a British citizen."

He thanked medical staff at University College hospital, the British government and the British people for their support.

Mr Litvinenko died in hospital just after 9pm yesterday. His condition had deteriorated rapidly after he suffered a heart attack and lost consciousness on Wednesday night.

Speaking outside the hospital this morning, his father, Walter Litvinenko, who was at his son's bedside when he died, described him as "very courageous".

"A terrible thing has happened here today," he said in Russian, dabbing tears from his eyes. "He was a very honest and good man and we loved him very much. And now he is not with us.

"He was very courageous when he met his death and I am proud of my son. It was an excruciating death ... but he never lost his human dignity.

"Marina and [Alexander] were the most wonderful couple. They were so happy here in London, but the long hand of Moscow got them here on this soil. This regime is a mortal danger to the world."

Since Mr Litvinenko became ill after a series of meetings in central London on November 1, his friends and family have insisted that he was the victim of a Moscow plot.

The former KGB agent was a fierce critic of Mr Putin's government and was investigating the murder of his friend, the Russian journalist Anna Politkovskaya, when he became ill.

But both the Kremlin and Russia's foreign intelligence service, the SVR, have denied any involvement. A Kremlin spokesman today described his death as "a tragedy".

Mr Litvinenko's friend Oleg Gordievsky, another former spy who defected to Britain, said today: "He was fighting against the evil forces in Russia, against the KGB, against the authorities which are suppressing democracy and liberal freedoms in Russia.

"He became a victim of ... revenge and malice of those forces in Russia."

One doctor said the type of poison used may never be known. Earlier suggestions that it was a heavy metal such as thallium or a radioactive substance were dismissed yesterday.

News of Mr Litvinenko's death spread as officials prepared for an EU-Russia summit in Helsinki, where questions about the former spy threatened to overshadow the main agenda.

If Russia is found to have had a hand in the poisoning, there could be serious diplomatic consequences. It would be the first such incident known in the west since the end of the cold war.

"We know he had a major dose," added HPA chief executive Professor Pat Troop, adding that it was unprecedented that "someone has apparently been deliberately poisoned by a type of radiation" in Britain.

Litvinenko, who died in a central London hospital late Thursday, would have had to have eaten, inhaled or taken it through a wound, she added, but said it was a police matter about how that was done and how the substance was obtained.

Staff at the HPA had worked through the night to assess the risks for people who came into contact with him since he first fell ill on November 1, and they were working closely with the two hospitals involved in his treatment.

Physical contact with Litvinenko himself would not pose any risks, but the dangers increased when there was contact with excretia from the body such as urine, faeces and to a lesser extent sweat, Cox said.

But he added: "The risk to those individuals from taking in contaminated blood, urine or faeces from Mr Litvinenko is very small and the risk we believe is insignificant."

All staff were being monitored as were the hospitals, the University College Hospital London and the Barnet and Chase Farm Hospital in north London.

Troop declined to speculate how many people might have been in touch with the dead agent. "We're talking minimum of 10s because he was in hospital for several weeks. During that time a number of staff looked after him," she said.

Asked about what caused Litvinenko's death, she added that was for the coroner to determine and their job was to assess the risk to others, including the wider public.

Cox said that he could not rule out that the polonium came from natural causes, but added that this was "unlikely."

A Kremlin spokesman on Friday said simply that Litvinenko's death was a matter for the British authorities and refused to be drawn on whether Moscow would investigate adamant claims from the deceased's friends that Russian secret services were involved.

"Death is always a tragedy. Now this is the turn of the investigation by relevant British services," the spokesman, Dmitry Peskov, said on the sidelines of an EU-Russia summit in Helsinki.

Britain's Guardian newspaper raised a number of possible lines of police inquiry, including self-poisoning and even that the former Russian Federal Security Services lieutenant-colonel's illness was a result of natural causes.

It also detailed the "polished public relations campaign" that swung into action after Litvinenko's health deteriorated last week, noting that the same agency is retained by his friend, the billionaire dissident Boris Berezovsky.

Berezovsky is wanted in Russia on fraud charges. An avowed enemy of Russian President Vladimir Putin, he has been granted political asylum in Britain.

Alex Goldfarb, who has been fielding media enquiries about Litvinenko throughout, is an executive director of Berezovsky's International Foundation for Civil Liberties, and holds a US passport.

Oleg Gordievsky, a former KGB colonel who defected from Soviet Russia to Britain in the mid-1980s, has also done the round of interviews, saying he had "no doubt" Russian secret services and Putin were behind his friend's death.

"He only had one enemy -- it was the head of the KGB, the KGB itself and Putin. He kept writing articles against Putin and against the KGB, particularly the head of the KGB... So, they decided to kill him," he told the BBC Thursday.

Litvinenko and his entourage have blamed Russia throughout because of the former secret serviceman's outspoken criticism of the Kremlin.

In his book "Blowing Up Russia: Terror from Within", Litvinenko alleged secret services set up the 1999 apartment block bombings which triggered the second Chechen war and propelled the then-little known Putin to power.

He was also investigating the death of the Russian journalist Anna Politkovskaya, a critic of her country's involvement in Chechnya who was shot at her central Moscow apartment building on October 7.

"This is what it takes to prove one has been telling the truth," Litvinenko is said to have told Nekrasov as his strength slipped away at University College Hospital London on Tuesday, The Times said Friday.

Security expert Glenmore Trenear-Harvey rejected the claims of politically-motivated killing, telling BBC radio that the threat to bilateral relations between Russia and Britain would be "far too great".

There was no evidence for the allegations, he added, highlighting the fact that police had classified the case initially as suspected poisoning but it was now being treated as an "unexplained death".

Litvinenko died at 9:21 pm (2121 GMT) Thursday after suffering a dramatic deterioriation in his condition throughout the day, including a cardiac arrest.

Earlier Thursday doctors ruled out an initial theory that the heavy metal thallium was responsible for his condition and that radioactivity was "unlikely". He first fell ill on November 1.

Echo Moscow radio on Friday quoted Goldfarb as saying that Litvinenko's burial date was not yet fixed because of the medical investigation, which could last several days.

At the same time, a rebel website Chechen Press said that Litvinenko had converted to Islam "some time ago" and would be buried in a Muslim graveyard.