THE former spy who blamed Russian president Vladimir Putin for his murder was poisoned with a lethal radioactive chemical, doctors have revealed.

Alexander Litvinenko accused president Putin of being behind his mysterious death in a posthumous letter released after he died in a London hospital last night.

Professor Pat Troop, from the Health Protection Agency, said traces of a deadly radioactive material were found in Mr Litvinenko's body.

He said he would have had to either eaten, inhaled or be given the high dose of the element, polonium 210, through a wound.

Police and health experts have begun searching various locations in the capital - including the two hospitals where he was treated - for radioactive contamination.

There are fears people who have come into very close contact with Mr Litvinenko, including healthcare professionals at UCLH and Barnet Hospital, could also be contaminated.

A post mortem has been delayed because of safety fears for the forensic scientists.

In a letter read out by his friend Alex Goldfarb outside University College London Hospital this morning, Mr Litvinenko sent a message to Mr Putin.

"You may succeed in silencing me, but that silence comes at a price," he said in a statement dictated two days ago.

"You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed.

"You have shown yourself to have no respect for life, liberty or any civilised value."

Addressing Mr Putin directly, he added: "You may succeed in silencing one man, but the 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, not only to me, but to beloved Russia and its people."

The 43-year-old ex-KGB officer lost the fight for his life at 9.21pm.

A statement from Scotland Yard said: "The matter is being investigated as an unexplained death. Enquiries continue into the circumstances surrounding how Mr Litvinenko became unwell."

The Russian had been in the hospital since November 17 after he was allegedly poisoned at a sushi bar in Piccadilly on November 1.

Friends and associates believe the 43-year-old was targeted because of his vocal opposition to president Putin. They claim he had been bugged for months by intelligence officers from the Russian Embassy in Kensington.

Prof Troop said people who came into close contact with Mr Litvinenko - including his wife and 12-year-old son - would be monitored.

"It is prudent to monitor as a precaution those people who came into direct and close contact with Mr Litvinenko to ensure there has been no cross contamination - agency staff are meeting with these people urgently," he said.

"There would be a potential radiological hazard to people who could have ingested or breathed in the contaminated body fluids, but this hazard is likely to be restricted to those who have had very close contact with Mr Litvinenko."

Barnet Hospital confirmed that the ward where Mr Litvinenko was treated had been emptied so it could be tested for radioactive contamination. Staff who treated Mr Litvinenko will also be checked.

Detectives are still searching for the two people whom Mr Litvinenko met the day he fell ill.

One is a Russian man whom he spoke to over a cup of tea at a London hotel and the other is Mario Scaramella, an Italian academic whom he met at the sushi bar.

Mr Scaramella, who has since gone into hiding, had emailed the former secret agent to say he had new information on the murder of Anna Politkovskaya, the 48-year-old investigative journalist shot dead in Moscow last month.

Mr Litvinenko told detectives he had arranged to meet an old friend, but was surprised to find another Russian man present.

The mystery man only introduced himself as "Vladimir" and revealed nothing about his identity.

Russian dissident Oleg Gordievsky, a former KGB colonel, told the BBC that the poisoning had been the work of the Russians.

He claimed the Russian security service had "sent a man with a poisonous pill to Britain put a pill into Mr Litvinenko's tea and killed him".

Both the Kremlin and Russia's foreign intelligence service have denied poisoning Mr Litvinenko.