England's participation in the football World Cup in Moscow could be in doubt if Russia is linked to the suspected poisoning of former double agent Sergei Skripal, the Foreign Secretary has suggested.
Boris Johnson has warned Russia that it will face a "robust" response from the UK if it is implicated in the incident, in which the former spy's daughter Yulia was also targeted, with the England football team's presence at this northern summer's tournament now in jeopardy.
Answering an urgent question in the House of Commons, Johnson told MPs that, should their suspicions be confirmed, Russia would face tougher sanctions as he labelled the Kremlin a "malign and disruptive force", the Daily Telegraph reports.
Asked whether the incident, which has sparked a major investigation involving counter-terror police, would lead to a breakdown in relations, Johnson said: "We will have to have a serious conversation about our engagement with Russia.
"For my own part, I think it will be very difficult to see how we...thinking ahead to the World Cup in July, I think it will be very difficult to imagine that UK representation at that event could go ahead in the normal way, and will have to think on that."
Allies of Johnson later claimed he was referring to UK "dignitaries and officials", rather than than the withdrawal of the England football team.
Whilst Johnson refused to discuss the ongoing operation, he said the discovery of Sergei Skripal and his daughter unconscious in a Salisbury street was "disturbing" and had "echoes" of Alexander Litvinenko's murder in 2006.
Attacking the actions of Vladimir Putin and the Kremlin's involvement in Ukraine and the annexing of Crimea, Johnson accused Russia of becoming a "malign and disruptive force", adding that it had been responsible for "igniting the flames of conflict".
Pledging to take a firm stance should Russian involvement be uncovered, he continued: "I can reassure the House that, should evidence emerge showing state responsibility, then Her Majesty's Government will respond appropriately and robustly.
"Though I am not now pointing fingers, I say to governments around the world, that no attempt to take innocent life on British soil will go unsanctioned or unpunished."
His comments to MPs came as it was reported 66-year-old Skripal was still providing help to MI6 while exiled in the UK after a spy swap.
Skripal gave occasional lectures on Russian spying to military academies in the UK and performed "consultancy" for MI6 as part of his resettlement package after being released in a spy swap.
The 66-year-old was also being paid an MI6 pension, the BBC's Newsnight reported.
The details of his life in Britain came as pictures were released that may show him minutes before he was allegedly poisoned.
CCTV appears to show Skripal and Yulia walking down a street in Salisbury.
The two are seen walking through an alley that connects Zizzi restaurant and the bench where they were found unconscious. The image was shown to the Telegraph by a local gym owner.
It came as the Kremlin denied any involvement.
A spokesman for the Russian Government said it was ready to cooperate if Britain asks it for assistance in investigating the incident.
Dmitry Peskov, the Kremlin's official spokesman, said that Russia was "open for cooperation" but had yet to be approached by investigators in the UK.
Regarding the CCTV footage, Cain Prince, the gym manager, said: "Police came in yesterday and reviewed the footage. These were the only people they took a photo of. Police said he was wearing a green coat. Freya, one of our employees, saw them on the bench just outside."
Freya Church, 27, who works at Snap Fitness gym opposite The Maltings, said she left work at 4pm on Sunday (Monday NZT) and saw Skripal and the younger woman on the bench just a few metres away.
"I was particularly worried about her," she told the Telegraph.
"She was slumped over on the man's shoulder. To be honest, I thought they might be homeless but they were perhaps better dressed.
"I just thought this is weird, especially as she was clearly quite a bit younger than him.
"She had a red bag at her feet. He was gesturing at the sky, doing some kind of movements with his hands.
"He was looking up and his eyes were glazed.
"There was no one else there near them at this point. No one was helping them."
Shown the CCTV footage of a couple walking past the gym towards the bench just 15 minutes earlier, Church said it was "definitely them, 100 per cent."
Destiny Reynolds, 20, who works at Ganesha handicrafts in The Maltings, witnessed the commotion.
She told the Telegraph: "I saw them both on the bench. They were rigid but then she was having some kind of episode.
"There was a big crowd around them. It was raining and people were covering them with umbrellas.
Then they put the woman on the floor in the recovery position."
Zizzi Salisbury has been closed as a precautionary measure, according to local police.
Wiltshire Police said: "Public Health England are aware of this and have reiterated that, based on the evidence to date, there is no known risk to the public's health."
Following the launch of a major investigation, the country's top counter-terrorism officer, Mark Rowley, warned that Britain must be "alive to state threats" as he revealed that counter-terror police were supporting the investigation.
Whilst Rowley refused to comment on the progress of the investigation, he stressed that British police were "very alert" to state threats.
"Clearly it's a very unusual case and the critical thing is to get to the bottom of what has caused this incident as quickly as possible," he told BBC Radio 4's Today programme.
"As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire Police to get to the bottom of that as quickly as possible.
"If you look back at other cases like (Alexander) 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."
An investigation by Buzzfeed raised questions over the deaths of 14 people in the UK in recent years, with US spy agencies suggesting the cases may be linked to Russia.
The report, published last year, claimed that a circle of nine wealthy businessmen, with connections to Russia, had died in suspicious circumstances but that investigators had shut down the cases.
It also revealed that US agencies had handed the Government intelligence on the Russian whistleblower Alexander Perepilichnyy, who died in 2012, which suggested that he was assassinated on the orders of the Kremlin.
Richard Walton, Scotland Yard's former terror commander, said that while police were "never complacent", investigating the cases was "very, very dangerous territory" and beyond the capabilities of local forces.
Asked about the cases today, Rowley admitted that there were deaths that had attracted "attention" in recent years.
However, he cautioned against jumping to conclusions before the full details emerged, adding: "I think we have to remember that Russian exiles aren't immortal, they do all die, and that can give rise to conspiracy theories.
"But likewise, we have to be alive to state threats, as illustrated by the Litvinenko case. We're very alert to that, which is why we'll throw all our technical resources at these sorts of cases to get to the bottom of whether there is any signs of foul play."
A former Russian intelligence agent with the FSB, Colonel Skripal was jailed in Moscow for spying for Britain was granted refuge in the UK in 2010 as part of a prisoner exchange.
Anna Chapman, a Russia-born secret agent who had acquired British citizenship and who was detained by US authorities, was sent back to Russia along with nine other agents.
Whilst Russia has already taken steps to distance itself from the incident, the emergence of footage from 2010 - showing Vladimir Putin describing Skripal and other spies exchanged in the swap as "traitors" - will fuel speculation of the state's involvement.
Speaking after the exchange, Putin said of the spies: "Traitors will kick the bucket, trust me.
"These people betrayed their friends; their brothers in arms. Whatever they got in exchange — those 30 pieces of silver they were given — they will choke on them."