Foreign Secretary Boris Johnson has vowed that Britain will mount a "robust" response if evidence of state responsibility emerges after a Russian double agent was left fighting for his life following suspected exposure to an unknown substance.
Sergei Skripal, 66, was found unconscious in Salisbury, Wiltshire, along with his 33-year-old daughter Yulia shortly after 4pm on Sunday. They are in a critical condition.
Addressing the Commons about the "disturbing incident", Mr Johnson noted that this case had "echoes" of the death of Alexander Litvinenko, a Russian dissident who was fatally poisoned in London in 2006.
He told MPs: "While it would be wrong to prejudge the investigation, I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's Government will respond appropriately and robustly."
In a fresh sign of the deterioration in relations between the countries, Mr Johnson also claimed Russia is "in many respects a malign and disruptive force".
As police mounted one of the most politically sensitive inquiries for years:
- The Russian Embassy said it was "completely untrue" to suggest the country's special services were involved
- Scotland Yard said the probe is being led by the counter-terrorism policing network because of its "specialist expertise" adding: "It has not been declared a terrorist incident and at this stage we are keeping an open mind as to what happened"
- A military research facility at Porton Down, Wiltshire, was believed to be involved in examining what could have caused the father and daughter to fall ill
- It was revealed that a "small number" of emergency services personnel, including police, were assessed immediately after the incident, and all but one have been released from hospital
- Prime Minister Theresa May and senior ministers were updated on the investigation at a meeting of the National Security Council
Mr Skripal was convicted in 2006 of passing state secrets to MI6 before being given refuge in the UK as part of a spy swap.
The former colonel in Russian military intelligence, who was sentenced to 13 years in prison, was among four convicts who were given pardons and one of two sent to Britain in 2010 in a deal that was said at the time to be the largest exchange since the Cold War.
He was found along with his daughter on a bench in The Maltings in Salisbury after police were called by a concerned member of the public at around 4.15pm on Sunday.
The pair did not have any visible injuries and were taken to Salisbury District Hospital, where they are being treated in intensive care for "suspected exposure to an unknown substance".
On Monday night officers "secured" a number of scenes - including the Zizzi restaurant on Castle Street and the Bishop's Mill pub in The Maltings.
A CCTV image of a man and woman walking steadily through an alleyway between the Zizzi restaurant and the bench near a shopping centre where Mr Skripal was found is believed to be of interest to police.
Mr Rowley confirmed his specialists were supporting the investigation.
He said: "If you look back at other cases like Litvinenko, if necessary we will bring that investigation into the counter-terrorism network. At the moment the key is, though, to get to the bottom of what caused this."
He added: "I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories.
"But likewise we have to be alive to the fact of state threats as illustrated by the Litvinenko case."
While the incident remains shrouded in mystery, it comes at a time when ties between Russia and the UK are under severe strain.
Former Scotland Yard counter-terror chief 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 bilateral relations.
"Relations that are already at breaking point. The UK cannot and will not tolerate state-sponsored terrorism of any kind."
The Kremlin said it did not have any information, adding that Moscow is "always open to co-operation".
Later, the Russian Embassy issued a statement saying it had not been provided with any official information about the incident.
Accusing the media of "launching a new phase of the anti-Russian campaign ongoing in the UK", the Embassy added: "Although UK law enforcement agencies have not given any substantive comments on this incident, media reports create an impression of a planned operation by the Russian special services, which is completely untrue.
"We believe that the British authorities and law enforcement bodies should step in immediately and inform the Embassy and the British society about the actual circumstances of this incident, so as to end the demonisation of Russia.
"With this in mind, the Embassy has turned to the Foreign Office for clarifications."