Moscow says it doesn't know what happened to former double agent Sergei Skripal, who is critically ill in a UK hospital after being exposed to an unidentified substance.
Skripal, 66, and his 33-year-old daughter Yulia were found unconscious on a bench in a shopping centre at Salisbury in England, where Skripal was granted refuge after a "spy swap" in 2010.
But the Kremlin says it stands ready to help British authorities if they ask for assistance.
In a conference call with reporters on Tuesday, Kremlin spokesman Dmitry Peskov called the incident a "tragic situation".
"Nevertheless, we don't have any information on what could have caused it and what this person was doing," he added.
Peskov said Moscow is "always open for cooperation".
When asked to respond to British media speculation that Russia had poisoned the ex-spy, Peskov said:
"It didn't take them long."
Speaking at The Security and Counter Terror Exposition in London, a former head of Scotland Yard's counter-terrorism command, Richard Walton, said: "The investigation must take its course but if this is state-sponsored terrorism and it looks entirely possible, then it will have grave consequences for UK Russia bi-lateral relations. Relations that are already at breaking point. The UK cannot and will not tolerate state-sponsored terrorism of any kind."
Foreign Secretary Boris Johnson told the British parliament that if any of the suspicions proved to be "well-founded" then the government would take "whatever measures we deem necessary to protect the lives of the people in this country, our values and our freedom."
Johnson said the government would wait for the outcome of the investigation to decide on the appropriate response.
Russia's embassy in London said on Tuesday it was seriously concerned what British media was reporting on the hospitalisation of former Russian agent Sergei Skripal and added the incident was being used to "demonise Russia".
"The British media are swiftly launching a new phase of the anti-Russian campaign," said a spokesman for the Russian embassy.
"The British authorities and law enforcement agencies must intervene immediately and inform the Embassy and British society about the real state of affairs in order to put an end to the demonization of Russia. In light of that, the embassy asked for the relevant explanations from the UK Foreign Office," the embassy's spokesman told TASS.
Marina, the widow of Alexander Litvinenko, who was poisoned 12 years ago in London, told Britain's Telegraph newspaper that she fears similarities to that murder.
The Kremlin has also repeatedly denied any involvement in the case of Alexander Litvenko, the ex-KGB agent killed with radioactive polonium-210 in London in 2006.
Litvinenko, an outspoken critic of Russian President Vladimir Putin who fled Russia for Britain six years to the day before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel.
Ben Emmerson, a lawyer for Marina Litvinenko, the widow of Alexander, said he wanted the police to fully investigate the incident, adding: "For now it looks very much like an FSB operation designed to punish a perceived traitor. If it looks like a duck and quacks like a duck then it probably is a duck.”
The Kremlin has always denied any involvement in Litvinenko's death.