Police are stepping up their inquiries into the death of the Russian ex-spy which is linked to radiation poisoning.
The post-mortem examination on Mr Litvinenko has been delayed
Alexander Litvinenko's death, first described by police as "unexplained", is now being treated as "suspicious".
Officers will fly to Moscow, while Russia's ambassador in London has been asked to provide any information which could help UK police.
Meanwhile UK minister Peter Hain has criticised Russia's "huge attacks" on liberty and democracy.
The Northern Ireland secretary told BBC News that "murky murders" had cast a shadow over Russian President Vladimir Putin's achievements in Russia.
He said Mr Putin's success "must be balanced against the fact that there have been huge attacks on individual liberty and on democracy and it's important that he retakes the democratic view."
Mr Litvinenko had been investigating the murder of a prominent Russian journalist, Anna Politkovskaya, before he fell fatally ill.
The BBC's diplomatic correspondent James Robbins said other ministers had been more cautious about "pointing a finger directly at the Russian state" over Mr Litvinenko's death, "still less President Putin himself".
The former agent's death has been linked to the presence of a "major dose" of radioactive polonium-210 in his body.
Radioactive traces were found at the Itsu restaurant in Piccadilly and the Millennium Hotel's Pine Bar and decontamination work has begun.
Hundreds of people called the NHS Direct hotline following Mr Litvinenko's death and a number were asked to submit urine samples for analysis. The results are expected later this week.
The Health Protection Agency has urged anyone else who visited the same London hotel or sushi bar on 1 November, when he met his contacts, to get in touch. But it added the risk to the public of exposure was low.
Home Secretary John Reid said the government was doing all it could to keep people informed about the situation.
He also told BBC Radio Clyde police were now treating Mr Litvinenko's death as suspicious, rather than "unexplained".
"As at this stage, they're saying to me that they now regard the death as suspicious. That wasn't the case yesterday [Saturday], for instance," Mr Reid said.
UK civil contingencies committee, Cobra, has met to discuss the case.
Friends have said Mr Litvinenko was poisoned because of his criticism of Russia.
In a statement dictated before he died at University College Hospital on Thursday, the 43-year-old accused Russian President Vladimir Putin of involvement in his death. He was known to be a fierce critic of him.
Mr Putin's spokesman Dmitry Peskov reiterated the Kremlin's earlier dismissal of allegations of involvement in the poisoning as "sheer nonsense".
The president himself has said Mr Litvinenko's death was a tragedy, but he saw no "definitive proof" it was a "violent death".
Andrei Nekrasov, who was close to Mr Litvinenko, said his friend had been a "strong man" who was left so debilitated by the rapid deterioration of his health that he was "reduced" to "screaming from pain".
Forensic tests were also carried out at the dead man's home
Speaking on BBC 1's Sunday AM show, Mr Nekrasov also said that he did not believe Mr Putin was "directly" responsible, although he thought "rogue elements" connected to the Russian leader were.
He said: "I think that Putin's orders on this are unlikely.
"Those rogue people are, in my opinion, a direct responsibility of Mr Putin. They are the result of the ideology of falsely understood nationalism which is now being injected into the Russian people."
Meanwhile, the Conservatives are to call for a Commons statement on Mr Litvinenko's death.
Shadow home secretary, David Davis, intends to raise the matter when MPs return to Westminster on Monday.
Mr Davis is expected to question ministers about the safety of Russian dissidents in the UK and to ask how polonium-210 was brought to the UK.
Mr Reid has rejected calls for a Commons statement, saying he would keep the situation under review.