Theresa May is visiting Salisbury after the nerve agent attack on a Russian former double agent and his daughter.
The prime minister is being briefed by public health experts, as the pair remain critically ill in hospital.
Mrs May says Russia is “culpable” for the attack and is expelling 23 Russian diplomats – prompting Moscow, which denies involvement, to say it will expel UK diplomats soon.
Defence Secretary Gavin Williamson said Russia should go away and shut up.
Russian former spy Sergi Skripal, 66, and daughter Yulia Skripal, 33, remain critically ill in hospital 11 days after they were found unconscious on a bench in Salisbury, Wiltshire.
Sergei Skripal, 66, and his daughter Yulia, 33, are in a “critical but stable” condition, Boris Johnson said
They had been poisoned with a nerve agent of a type developed by Russia, called Novichok, the UK government says.
A sample is to be sent to the Organisation for the Prohibition of Chemical Weapons – the independent international body set up to stop chemical warfare – for analysis.
Russian Foreign Minister Sergei Lavrov reportedly said the allegations against Russia were “absolutely boorish” and said the UK’s approach was partly prompted by its Brexit problems.
Mrs May is speaking with emergency services, local businesses and members of the public during her visit on Thursday.
The PM, is also meeting Public Health England officials, expelled the Russian diplomats on Wednesday after Moscow refused to meet the UK’s deadline to explain the use of the nerve agent.
She said there was “no alternative conclusion” than to believe Russia was “culpable” for the poisonings.
He said there was “no doubt the attack came from Russia”.
Meanwhile, Foreign Secretary Boris Johnson told the BBC the attack was Russia’s “way of saying to people this is what happens to people who stand up to our regime”.
The foreign secretary tells Today the Russia tensions aren’t a return to the Cold War
Mr Johnson said: “Now is the moment for [President Vladimir] Putin to jam the lid down and send a signal to people: ‘You do this, you’re going to die.'”
He said the UK’s allies had been “steadfast in their support” and Foreign Secretary Boris Johnson said he was “heartened” by the response around the world.
The White House offered full support to its closest ally and accused Russia of undermining the security of counties worldwide.
Meanwhile, French President Emmanuel Macron’s office released a statement saying France agrees with the UK’s claim and there was “no other plausible explanation” than Russia’s involvement.
Mr Skripal is a retired colonel in the Russian military intelligence service. He was jailed by Moscow in 2006 for secretly working for Britain’s MI6 but was later released and allowed to come to the UK.