The UK Foreign Office has condemned as "unacceptable" Russia's refusal to extradite Andrei Lugovoi over the murder of Alexander Litvinenko.
Mr Litvinenko died in a London hospital in November 2006
Russia has said its constitution did not allow a citizen to be extradited.
A Foreign Office spokeswoman said a reply would be considered "with the seriousness that it deserves".
Mr Litvinenko, a former KGB agent, died of exposure to radioactive polonium-210 in London last November, and Mr Lugovoi denies involvement.
The Crown Prosecution Service said in May that it wanted Mr Lugovoi, himself an ex-KGB officer, to face trial in the UK.
The Foreign Office spokeswoman added: "We've consistently said that the murder of Mr Litvinenko is a serious criminal matter.
"Hundreds of British citizens and visitors to the capital were put at risk."
Russian prosecutors formally announced their refusal to extradite Mr Lugovoi to the UK on 5 July.
A Downing Street spokesman said the decision was "extremely disappointing".
He added: "Russia is an important partner on many issues and we continue to seek a constructive relationship with them - but we need to carefully consider our range of co-operation."
Under the Council of Europe European Convention on Extradition 1957, the Russians have the right to refuse the extradition of a citizen.
The Convention gives the UK the right to request that the investigation be taken on by the Russian authorities.
However, Sir Ken Macdonald, director of public prosecutions, rejected an offer from Moscow to try Mr Lugovoi in Russia.
"The allegation against Mr Lugovoi is that he murdered a British citizen by deliberate poisoning and that he committed this extraordinarily grave crime here in our capital city," Sir Ken said.
"The appropriate venue for his trial is therefore London."
The Ministry of Justice said there were no circumstances in which a trial could be called when the defendant was not in the UK's jurisdiction and was unlikely ever to enter.