Johnson said the government had proof that Novichok was being stockpiled by Russia. He said: "We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination but has also been creating and stockpiling Novichok itself." The nerve agent used in the attempted assassination of Sergei Skripal and Yulia Skripal was probably delivered though his car's ventilation system. Sources have said that intelligence officials "now have a clearer picture of just how the attack was conducted". According to reports, security agencies now believe the toxin was used in a "dust-like powdered form" and that it circulated through the vents of Sergei Skripal's BMW.

Citing three intelligence officials, ABC News in the US said that the Novichok agent is likely to have been delivered in a powdered form. "It is a Cold War substance, something they [Russia] claimed never to have," an intelligence official said. A chemical weapons expert has said it would need just 5 microns, or 0.005 millimetres, of the agent placed in the car's air vents or air con system for it to be effective. Friends of two of the dead Russian dissidents said it was vital their bodies be exhumed. The Home Office, which has ordered a review, declined to "give more details at this stage". The Metropolitan Police also refused to comment. Self-exiled oligarch Boris Berezovsky, described as an enemy of Vladimir Putin, pictured in London in 2010. Credit:AP

One of the dead, Badri Patarkatsishvili, was 52 when he was found at his mansion in February 2008 in Leatherhead, Surrey, where he had been in exile since 2001. He was a business partner of Boris Berezovsky, Mr Putin's arch enemy, who was found dead at his Surrey home five years later. Nikolai Glushkov, 68, a third business partner, was murdered at his home in New Malden, south London, a week ago. A friend who did not wish to be named for fear of reprisal said: "The Home Office must consider exhuming Badri's body. They never did the toxicological report on Badri. Surrey police just said he had a bad heart and had a heart attack. But I had seen him before his death and he was absolutely fine. The next day he was gone." Russian President Vladimir Putin, pictured on Sunday after his re-election, dismissed questions about Russia's alleged role on the poisonings of ex-spies and dissidents. Credit:AP It was also found that Surrey Police did not test for a nerve agent in the case of Alexander Perepilichny, 43, who collapsed while jogging near his home on the St George's Hill private estate in 2012. Perepilichny had provided evidence of fraud perpetrated by Kremlin-linked officials. Police were unable to determine the cause of his death but said it appeared not to be suspicious.

Loading Loading However, an associate yesterday called for his body, too, to be exhumed. "He was never tested for nerve agent," said the source. Amber Rudd, the Home Secretary, has ordered a fresh inquiry into 14 deaths in total, including those of Perepilichny, Patarkatsishvili and Berezovsky. Yvette Cooper, the Labour chairman of the home affairs committee, said it was vital the deaths were reinvestigated fully. She said: "It is clear that further criminal investigations are needed into the activities of the Russian state on our soil."

On Sunday night Putin thanked supporters outside the Kremlin after winning his fourth term as president. Asked by journalists about the poisoning of Skripal, Putin said it was "total nonsense that someone in Russia would have allowed this" before the election. Tensions between Moscow and London further escalated on Sunday when Vladimir Chizhov, Russia's ambassador to the EU, suggested the nerve agent used in the attack on the Skripals was made at Porton Down, "only eight miles from Salisbury". Putin said he first learned about the poisoning, which he called a "tragedy", from the news. His initial reaction, he said, was that people would have died on the spot if it had really been a military-grade poison. He also denied that Russia had any such substance.

"We destroyed all of our chemical weapons under the control of international observers," Putin said, speaking at a late-night news conference to celebrate his election victory. "Moreover, we were the first to do this, unlike some of our partners who promised to do this but have yet, unfortunately, to keep their pledge." In an interview with the Russian RT news channel on Friday, meanwhile, Russia's ambassador to Britain, Alexander Yakovenko, questioned whether the Skripals were sick at all. Loading Replay Replay video Play video Play video "All the investigation about Skripal is classified," he said. "Nobody even saw the pictures of these people in the hospital — whether they are alive, or maybe they are just in good health. Nobody talked to the doctors. You know, there is absolutely no transparency in this case." He also said that British leaders might have concocted the Skripal poisoning as a way to distract from the frustrating logjam over Britain's withdrawal from the European Union.

And in his first intervention, while voting in the Russian elections, Mikhail Gorbachev, the last leader of the Soviet Union, said the attack was "disgusting" but stopped short of blaming Putin. Telegraph, London, New York Times