The attempted murder of ex-Russian spy Sergei Skripal and his daughter, Yulia, using a nerve agent was a “brazen and reckless attack”, Amber Rudd has said.
Mr Skripal and his daughter are still critically ill after being found collapsed on a bench in Salisbury city centre on Sunday.
Counter-terrorism officers are working to find the origin of the nerve agent.
A police officer, who was in intensive care, is now “stable and conscious”, Wiltshire’s chief constable said.
Addressing the House of Commons, the home secretary said the attack was “attempted murder in the most cruel and public way”.
Ms Rudd told MPs it was an “outrageous crime”, adding that the government would “act without hesitation as the facts become clearer”.
She refused to speculate on whether the Russian state might have been involved in the attack, saying the police investigation should be based on “facts, not rumour”.
However, she said the government was committed to bringing the perpetrators to justice “whoever they are and wherever they may be”.
Prime Minister Theresa May told ITV News that “if action needs to be taken then the government will do that”.
“We will do what is appropriate, we will do what is right, if it is proved to be the case that this is state-sponsored,” she said.
Earlier, Ms Rudd said the nerve agent used in the poisoning was “very rare”.
Responding to the home secretary’s speech, the UK’s Russian embassy Tweeted that it “totally” agreed with her vow to first establish evidence and then publish official conclusions.
Police said government scientists had identified the nerve agent used, but would not make that information public at this stage.
The source familiar with the investigation told the BBC it was likely to be rarer than the Sarin gas thought to have been used in Syria and in an attack on the Tokyo subway in 1995.
And it was said not to be VX – the nerve agent used to kill the half brother of the North Korean leader Kim Jong-un in Malaysia last year.
Mr Skripal, 66, was convicted of passing secrets to MI6 but was given refuge in the UK in 2010 as part of a “spy swap”.
It is known that he and his 33-year-old daughter had visited the Mill pub and Zizzi restaurant in Salisbury on Sunday afternoon, before they were found collapsed on a bench near the Maltings shopping centre.
A witness, who saw the pair at the restaurant, told the BBC Mr Skripal was acting “very strange” and was “very agitated”.
“He seemed to lose his temper… and he just started screaming at the top of his voice, he wanted his bill and he wanted to go.”
Police have yet to say if they know how and where the poison was administered.
Defence Secretary Gavin Williamson said Russia was becoming an “ever greater threat”, amid speculation the attack could have some element of state involvement.
“Russia’s being assertive, Russia’s being more aggressive, and we have to change the way that we deal with it because we can’t be in a situation in these areas of conflict where we are being pushed around by another nation,” he told ITV’s Good Morning Britain.
Police said they wanted to speak to anyone who was in the centre of Salisbury on Sunday afternoon.
They are particularly keen to hear from people who ate at Zizzi or drank in the Mill pub between 13:00 and 16:00 GMT.
Both sites remain closed to the public.
BBC correspondent Leila Nathoo said there had been “a flurry of activity” outside Mr Skripal’s Salisbury home at lunchtime.
She said ambulances and an incident response team were in attendance and a police tent had been erected outside the house.
Part of a business park in nearby Amesbury also remains cordoned off, while the graves of Mr Skripal’s wife and son at a Salisbury cemetery have also been taped off by police.
Speaking on Wednesday, Assistant Commissioner Mark Rowley said: “This is being treated as a major incident involving attempted murder, by administration of a nerve agent.
He said Mr Skripal and his daughter were “targeted specifically”.
“Our role now of course is to establish who is behind this and why they carried out this act,” he added.
Nerve agents are highly toxic chemicals that stop the nervous system working and shut down bodily functions.
They normally enter the body through the mouth or nose, but can also be absorbed through the eyes or skin.
Mr Rowley said there was no evidence of a widespread health risk to the public.
Hundreds of detectives, forensic officers, analysts and intelligence officers are working on the case, which has drawn comparison with the killing of Russian dissident Alexander Litvinenko with radioactive polonium-210 in London in 2006.
A public inquiry concluded his death was probably carried out with the approval of President Vladimir Putin.
Foreign Secretary Boris Johnson told MPs on Tuesday the UK would respond “robustly” to any evidence of Russian “state responsibility” in the Skripal case.
Russia has insisted it has “no information” about what could have led to the incident, but is open to co-operating with British police if requested.
Russian Foreign Ministry spokeswoman Maria Zakharova said foreign media had used the case as part of an anti-Russian campaign.