Small traces of a radioactive substance have been found at the British embassy in Moscow following a precautionary check, the UK Foreign Office has said.
Mr Litvinenko died on 23 November in a London hospital
But officials said the levels of radiation found would not pose a risk to public health.
It comes as British police said they were treating the death of ex-Russian spy Alexander Litvinenko as murder.
The former KGB agent's death on 23 November, in London, has been linked to the highly toxic isotope polonium-210.
"Detectives in this case are keeping an open mind and methodically following the evidence," Scotland Yard said in a statement.
"It is important to stress that we have reached no conclusions as to the means employed, the motive or the identity of those who might be responsible for Mr Litvinenko's death."
The British embassy announced on 4 December it would test one of its rooms as a precaution, after former KGB bodyguard Andrei Lugovoi visited the building to deny any involvement in the poisoning of Mr Litvinenko.
Mr Lugovoi and another Russian businessman reportedly met with the former spy in London on 1 November.
Earlier, Mario Scaramella, the Italian academic who met the ex-Russian spy Alexander Litvinenko on the day he fell ill, was discharged from hospital in London.
Preventative checks were carried out at the British embassy
Mr Scaramella was under observation after testing positive for polonium-210.
He had a meeting with Mr Litvinenko on 1 November at a sushi bar in central London.
A spokesman for University College Hospital said Mr Scaramella was not showing any symptoms of radiation poisoning.
The Health Protection Agency also confirmed that staff at the restaurant where the pair met had all tested negative for radiation.
Meanwhile, Dmitry Kovtun, a business associate of Mr Lugovoi, is reported to have been interviewed by British police in Moscow.
The pair met Mr Litvinenko on 1 November at the Millennium Hotel in central London.
Mr Lugovoi is also expected to talk to British detectives in the Russian capital, his business associate Vyacheslav Sokolenko said.
Mr Sokolenko confirmed he was in London with Mr Lugovoi on 1 November, but said he did not meet or know Mr Litvinenko.
He said the meeting would be at the clinic where he says Mr Lugovoi is undergoing medical checks.
Russian officials are expected to conduct the interviewing of Mr Lugovoi on Wednesday but British detectives will be in attendance.
"If they show me a list of people that they want to meet and if there are names missing on that list, names that I believe would be interesting to propose to them, then I certainly will," Mr Lugovoi previously told NTV television.
Nine Metropolitan police officers are currently in Moscow but have had restrictions placed on their investigations into Mr Litvinenko's death by the authorities.
Russia's chief prosecutor, Yuri Chaika, said his own officers would be conducting any witness interviews, British officers could not arrest Russian citizens and suspects would not be extradited to Britain.
Scotland Yard's anti-terrorism unit has not confirmed who officers will be meeting.
British police launched their investigation after Mr Litvinenko, 43, died in a London hospital on 23 November.
Tests have been carried out at a number of venues the ex-spy visited in London on 1 November.
Earlier, Mr Scaramella said he had received a "general alert" about the former spy's security.
In an interview from his hospital bed he told CNN he had travelled to London for a conference, but had changed his plans to discuss the threat with Mr Litvinenko .
He said had received e-mails claiming that both he and Mr Litvinenko were "under the special attention of hostile people", but neither man believed the threats were real.
When asked what kind of people would be targeting him, he said: "People linked with some clandestine organisations, not directly under control of Russian establishment but from Russia."
Friends believe Mr Litvinenko was poisoned because of his criticisms of the Russian government, but the Kremlin has dismissed suggestions it was involved in any way as "sheer nonsense".