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.