He said police would not give information about the exact substance at this stage. They are now tracing CCTV and other sources to build a timeline of the victims’ movements, to try to establish the attackers' identity and motive.

The Guardian reported the poison may have been a VX or sarin nerve agent, or a related substance.

Sarin is a toxin 26 times more deadly than cyanide, used most notoriously in the 1995 Tokyo subway attack. More recently it has reportedly been used by the Syrian government against rebels.

In 2002 an Arab-born terrorist in Chechnya died after opening a letter laced with sarin – a killing that media reports linked to a double agent working for the Russian Federal Security Service.

VX nerve agent is also reportedly part of Russia’s arsenal though other countries also have stores: it was believed to be used in the murder in 2017 of Kim Jong-nam, half-brother of North Korean leader Kim Jong-un.

Skripal and his daughter have been in hospital since they were found slumping into unconsciousness on a bench outside The Maltings shopping mall. Counter-terror police were called in on Tuesday to lead the investigation.

Skripal had been jailed in Russia in 2006 for treason after passing on information including the identity of undercover Russian agents to MI6, however he was pardoned in 2010 and sent to the UK along with three other men in the biggest spy swap since the Cold War.

Experts have pointed to Russia as having the motive and capability – as well as the past history – of making such an attack.

One Wednesday afternoon in Salisbury,  police, firefighters and paramedics swooped on a care home next door to the restaurant Zizzi, which had already been sealed off and searched as part of the investigation.

One woman was taken to hospital amid unconfirmed reports she had a panic attack in response to the raid.

Scientists at the UK's secret weapons research facility at Porton Down, near Salisbury, have been examining the substance thought used to poison Skripal.

Detectives were reportedly examining whether Skripal was sprayed with poison on the street, or if his drink was spiked at the pub.

After chairing a meeting of the government's emergency committee Cobra, Home Secretary Amber Rudd said authorities “need to keep a cool head and collect all the evidence we can’.

"We need to make sure that we respond not to rumour but to all the evidence they collect and then we will have to decide what action to take," she said.

She said the police investigation was likely to be a long one.

Her comments was interpreted as a rebuke to British Foreign Secretary Boris Johnson, who on Tuesday warned Russia there would be a “robust” response if they were proven responsible for the attack.

“No attempt to take innocent life on UK soil will go either unsanctioned or unpunished,” Johnson told the House of Commons, suggesting the UK would consider sanctions or other diplomatic action related to this summer’s FIFA World Cup to be held in Russia.

However he added he did not want to pre-judge the investigation.

Russia’s London embassy denied it had been a “planned operation” by the Russian specials services, and accused the British press of demonising Russia. Russian Foreign Ministry spokeswoman Maria Zakharova said Johnson’s comments were “wild”.

Relatives of Skripal, a former Russian colonel, told the BBC Russian Service that he believed the country's special services might come after him at any time.

The BBC reported Skripal’s only friends in the UK were in the British intelligence services, though he also joined his local railway club and gave lectures at military academies.

His wife, elder brother and son all died in recent years. His daughter was visiting him from Moscow, where she lives.