Counter-terrorism police have opened an investigation into the “unexplained” death on British soil of an arch enemy of Vladimir Putin, just eight days after the nerve gas assassination attempt on a Russian double agent.

Nikolai Glushkov, 68, the right-hand man of the deceased oligarch Boris Berezovsky, Mr Putin’s one-time fiercest rival, was found dead at his London home on Monday.

A Russian media source said Glushkov, the former boss of the state airline Aeroflot, who said he feared he was on a Kremlin hit-list, was found with “strangulation marks” on his neck.

The inquiry into Glushkov’s death was announced hours before a midnight deadline for the Kremlin to explain how Russian-made nerve agent came to be deployed in the assassination attempt on the double agent Sergei Skripal in Salisbury. The midnight deadline passed with no sign of response from Russia.

Russia appeared to suggest it would be unwise for Britain to provoke a fellow nuclear power and threatened to retaliate against sanctions, which Theresa May is expected to announce on Wednesday.

A foreign ministry spokesman said: “Any threats to take ‘sanctions’ against Russia will not be left without a response. The British side should understand that.”

Mrs May gained the support of Western leaders including Donald Trump and Angela Merkel for reprisals against the Putin regime that will include sanctions and the expulsion of spies based in the Russian embassy in London.

The home in New Malden credit: Ben Stevens/ i-Images Picture Agency

The White House issued a statement saying the US “stands in solidarity with its closest ally” and condemning the use of “heinous weapons in flagrant violation of international norms”.

A Downing St spokesman added: “President Trump said the US was with the UK all the way, agreeing that the Russian Government must provide unambiguous answers as to how this nerve agent came to be used.”

Mr Trump’s unequivocal support will have been welcomed, with the White House previously having refused to blame Russia. Earlier in the day, Mr Trump had sacked Rex Tillerson, his secretary of state, a day after the US diplomat had said the Novichok nerve agent “clearly came from Russia”.

The Salisbury inquiry widened on Tuesday as police said 38 people had been treated, prompting fears the Novichok nerve agent could have spread across the city.

Col Skripal, 66, and his 33-year-old daughter Yulia Skripal remain critical in intensive care while Nick Bailey, the Wiltshire police detective who went to the scene, is seriously ill but stable.

Nikolay Glushkov (left), Berezovsky and Badri Patarkatsishvili - all three are now dead

Firemen in chemical protection suits examined and removed material just feet from the bench where the Skripals collapsed, as the search for clues to the attack continued.

Boris Johnson, the Foreign Secretary, warned Russia not to underestimate British outrage at the attack and refused to rule out a retaliatory cyber strike.

The death of Glushkov came as Amber Rudd, the Home Secretary, announced a joint MI5 and police inquiry into the deaths in the UK of at least 14 people, including Berezovsky, said to have fallen foul of Mr Putin and Russian crime gangs linked to the Kremlin.

An anonymous acquaintance of Glushkov told Russia’s Kommersant newspaper that signs of strangulation had been found on the body, which was found by his daughter at his home in New Malden in south London on Monday evening. It was unclear whether the death was a result of suicide or murder, the paper’s source said.

View more!

Alex Goldfarb, a Russian dissident and friend of Glushkov, said: “It clearly looks suspicious in the wake of the poisoning of Mr Skripal. He (Glushkov) was a public figure in Russia and he was one of the closest partners of Mr Berezovsky.”

“His death will be perceived in Russia as yet another victory for Mr Putin in getting to fugitives.”

Marina Litvinenko, widow of Alexander Litvinenko, the former FSB spy poisoned by the radioactive substance polonium in 2006, said she was a good friend of Glushkov and upset by his death. “We have to wait and see exactly what has happened,” she said.

Glushkov had claimed that Berezovsky, who was found hanged in his bathroom, had been murdered on Mr Putin’s orders. Glushkov had claimed to have seen a scarf close to Berezovsky’s body, and said in one interview: “There were traces of him being strangled around the neck.”

Suspicions also surround the death of another of Glushkov and Berezovsky’s friends, Badri Patarkatsishvili, 52, a Georgian who died at his home in Surrey of an apparent heart attack in 2008. Andrey Lugovoy, who is blamed for the murder of Litvinenko, was at one time Patarkatsishvili’s chauffeur and security adviser to Glushkov. He is now a Russian MP and supporter of Mr Putin.

View more!

Lord Bell, the public relations guru and friend of all three men, said he was “not surprised” at Glushkov’s death. But he urged caution, saying Glushkov had suffered from a blood disorder as a result of years spent in Russian jails before his final release and exile to Britain in 2006. He was convicted again in absentia last year but Britain refused to extradite him. He had faced death threats and once been told he would be “run over by a truck”.

On Tuesday night two forensic tents stood outside the Thirties terraced house, where Glushkov lived alone with his dog for the past three years.

The Metropolitan Police described the death as “unexplained” and said in a statement that an investigation was under way.

The death of Glushkov will ratchet up tensions further in the middle of a diplomatic crisis over the chemical weapons attack on Col Skripal.

Forensic team tents outside the suburban property where Nikolai Glushkov lived credit: Ben Stevens/ i-Images Picture Agency

Russia has denied any involvement while Maria Zakharova, its foreign ministry’s spokesman, appeared to suggest it would be unwise for Britain to provoke a nuclear power.

As Mrs May’s deadline came closer, Miss Zakharova referenced Mr Putin’s state of the nation address in which he spoke of building new advanced nuclear weapons, adding: “After what the president said no one can go into the parliament of his country and say ‘I’m giving Russia 24 hours’. What’s that meant to mean?”

Sergei Lavrov, Russia’s foreign minister, demanded Britain provide it with a sample of the nerve agent so it could carry out independent tests.

Bereaved friends speak of his life and work

Vasily Trunin, a friend and apparent former colleague, wrote on Facebook: "Every time we get on a new, up-to-date plane made by Aeroflot, one of the best airlines in Europe, we can remember Kolya (Nikolai) Glushkov, who was one of the few people who managed to pull an airline out of the Soviet backwater.

"A shame, a great shame. He was a good friend."

He told the Telegraph that he met the former Aeroflot manager in London and that he had been a close friend of Mr Berezovsky and Badri Patarkatsishvili.

"I saw Boris and Badri a lot at that time and observed their friendship with Nikolai. He really was a very good friend of theirs."

Nikolai Glushkov before he left Russia

Paying tribute to Mr Glushkov, he said: "I heard many positive reviews about his organizational skills, his understanding of building production processes."

Another friend wrote that Mr Glushkov was the "best person [he] ever worked for".

Alexander Goldfarb, a Russian-American activist who has campaigned for justice for Alexander Litvinenko, wrote that he was saddened by the death.