Boris Johnson has said Vladimir Putin threatened to kill him with a "missile," and added it "would only take a minute".
Following his visit to Kyiv in early February last year, Mr Johnson said he had a "very long, most extraordinary call" with the Russian president.
After the then prime minister described the likely sanctions response to a Russian invasion of Ukraine, Putin replied: "Boris, I don't want to hurt you, but with a missile, it would only take a minute," Mr Johnson recalled.
The revelations come in a new three-part documentary series titled Putin vs the West which is to be shown on BBC Two and BBC iPlayer.
The programmes, which start on Monday, chart how the West has struggled for over a decade to deal with Putin as he grew in power on the world stage.
The series shows how publicly the Russian president maintained that his country had no plans to invade Ukraine, even as a steady stream of Russian battalions was heading to the border.
Mr Johnson says he warned Putin there would be much tougher Western sanctions if he invaded Ukraine and said Russian aggression would only lead to the West intensifying its support for Ukraine, leading to "more Nato, not less Nato" on Russian borders.
"He said, 'Boris, you say that Ukraine is not going to join Nato any time soon. […] What is any time soon?' and I said 'Well it's not going to join Nato for the foreseeable future. You know that perfectly well,'" Mr Johnson said of the crucial phone call with Putin.
"He sort of threatened me at one point and said, 'Boris, I don't want to hurt you, but with a missile, it would only take a minute', or something like that'.
"I think from the very relaxed tone that he was taking, the sort of air of detachment that he seemed to have, he was just playing along with my attempts to get him to negotiate," Mr Johnson said.
Russia claimed Ukrainians would 'welcome them'
The programme also details the last-minute attempt by Ben Wallace, the Defence Secretary, to stop Russia's war on Ukraine.
Less than two weeks before the Feb 24 attack, Mr Wallace visited Moscow, accompanied by the Telegraph.
Greeted by Russia's minister of defence Sergei Shoigu, and the head of Russia's armed forces, General Valery Gerasimov - the latter of whom has since been promoted by Putin to overall commander of the war in Ukraine - Mr Wallace said the Russians were adamant the Ukrainians "won't fight" and that they "will welcome them".
"I remember saying to Minister Shoigu 'they will fight' and he said, 'My mother is Ukrainian, they won't!'," the Defence Secretary said, adding: "He also said he had no intention of invading".
"It was the fairly chilling but direct lie of what they were not going to do that I think, to me, confirmed they were going to do it.
"I remember as we were walking out General Gerasimov said, 'Never again will we be humiliated. We used to be the fourth army in the world, we're now number two. It's now America and us.' And there in that minute was that sense of potentially why [they were doing this]," Mr Wallace said.
In the documentary Volodymyr Zelensky, Ukraine's president, said he complained to Jens Stoltenberg, Nato's Secretary-General, that his country was not being taken seriously as a prospective member of the 30-nation alliance.
"I told him: 'Jens, I want to join Nato, do you see us in Nato?' Because nothing would defend our country except for actual membership," Mr Zelensky said.
"I said: 'It's just unfair and not nice. You don't see us as equals.' I told him that our army is ready, our society is ready, and I believed that Nato is not ready.
"If you know that tomorrow Russia will occupy Ukraine, why don't you give me something today I can stop it with? Or if you can't give it to me, then stop it yourself."
All three episodes of Putin vs the West are available on BBC iPlayer. The first episode airs on BBC Two at 9pm on Monday.