Any involvement by the Russian state in the death of Alexander Litvinenko could have "enormous implications" for UK-Russian relations, David Davis has warned.
The shadow home secretary acknowledged the need for caution in attributing blame for the death of the former spy in London last week, but said it was "incredibly important" to resolve the issue.
He was speaking in response to John Reid's statement in the House of Commons this afternoon, in which the home secretary confirmed that several people were being tested for traces of the radioactive material that killed Mr Litvinenko.
Mr Davis said: "If it was, given that a British citizen under British government protection may have been murdered on British soil, this has enormous implications for the relationship between the United Kingdom and Russia.
"If it was not, it raises the almost equally disturbing prospect that such sophisticated capabilities are available to criminals in the United Kingdom, and possibly therefore to other groups."
Liberal Democrat home affairs spokesman Nick Clegg also questioned whether there had been any attempt to contact other Russians who have been granted asylum in the UK and who had anti-Kremlin views similar to Mr Litvinenko.
However, Mr Reid urged MPs to exercise "caution" in speculating who was behind the death while the investigation was ongoing. He also noted that police were not even sure it was murder, saying only that it was "suspicious".
He confirmed that last week there had been a meeting of Cobra, the cabinet emergency committee, but said this was more to do with concerns about public health than fears that Mr Litvinenko had been murdered.
"As far as the preliminary contacts with the Russian government are concerned, as I explained we have made plain to them that if it were required, we would expect them to give us all possible assistance," Mr Reid said.
The Foreign Office called in the Russian ambassador on Friday to discuss this cooperation, he said, and "as far as I'm aware they have indicated this would be the case".
Investigations by the Health Protection Agency (HPA) found traces of the radioactive polonium-210 (Pol-210) that killed Mr Litvinenko in his north London home in Muswell Hill and a restaurant and hotel in central London.
Following an appeal, three people have contacted NHS Direct as having been in one of these places and are now undergoing tests. However, Mr Reid stressed the "very precautionary nature" of these, and said there was no need for public panic.