Counter-terror police are to travel to Russia as part of the inquiry into the death of Russian former agent Alexander Litvinenko, the BBC has learned.
Mario Scaramella met Mr Litvinenko the day he fell ill
It comes as Home Secretary John Reid said the inquiry into Mr Litvinenko's poisoning would expand beyond Britain.
Mario Scaramella - an Italian contact of Mr Litvinenko who is currently in hospital - is said to be "well".
Traces of radioactive polonium-210 have been found in his body. He met Mr Litvinenko the day the ex-spy fell ill.
The BBC's Daniel Sandford said it was understood nine officers from the Metropolitan Police's counter-terrorism command could travel to Russia as early as Monday.
The specialist unit - which was launched in October to meet terrorist threats - is heading the investigation into the former KGB agent's poisoning.
News of the visit came after Mr Reid said the investigation was set to widen.
Discussing various aspects of Mr Litvinenko's death, Mr Reid told Sky News: "Over the next few days I think that all of these things will widen out a little from the circle just being here in Britain.
"Tomorrow I will be at the European Council and I will certainly be sharing information and getting what we can from European counterparts.
"The health authorities are already starting to liaise with our European colleagues and the police will follow wherever this investigation leads; inside or outside Britain."
Shadow home secretary David Davis welcomed news that the investigation was expanding.
He told BBC One's The Politics Show: "I think it's a good thing, I think it's very important that no channel is left unpursued, that this investigation goes right to its limit wherever that may be and that limit should not be a diplomatic limit, it should be the limit of the evidence."
There has been no change in the condition of Mr Scaramella, who is being monitored at University College Hospital, London.
Doctors said the academic - who was one of the last people to meet Mr Litvinenko before his death last month - was "well" with "normal" test results.
Forty-three-year-old Mr Litvinenko died on 23 November
He is said to be displaying no symptoms of radiation poisoning.
Mr Scaramella's lawyer Sergio Rastrelli told Channel 4 News his client was "clearly worried".
"The doctors said he has definitely been contaminated with polonium but it's not radioactive," he said.
"So he has ingested or inhaled it, but in [an] extremely minimal dose, far less than that with which Litvinenko was poisoned."
Mr Litvinenko's death is being linked to the discovery of polonium-210 in his body.
Friends believe he was poisoned because of his criticisms of the Putin government.
On Saturday, airline Easyjet said Mr Scaramella had flown on flight 3506 from Naples to Stansted, Essex, on 31 October and also on flight 3505 from Stansted to Naples on 3 November.
But the Health Protection Agency (HPA) said it had no "public health concerns" about those flights.
Transport Secretary Douglas Alexander also moved to reassure the public and said the government did have measures in place at airports to detect "radioactive materials".
The Health Protection Agency said just over 3,000 people had now called the NHS Direct line in the wake of the radiation scare in the UK, with 179 being followed up for further investigation.
Twenty-seven people were referred as a precaution to a specialist outpatient clinic for radiological exposure assessment.
A total of 70 urine samples, mainly from medical staff and ambulance workers, have been tested and found negative.