Boris Johnson has said Vladimir Putin threatened him with a missile strike in an "extraordinary" phone call in the run-up to Russia's invasion of Ukraine.
The then-prime minister said Mr Putin told him it "would only take a minute".
Mr Johnson said the comment was made after he warned the war would be an "utter catastrophe".
The claim is made in a BBC documentary on Mr Putin's interactions with world leaders over the years. The Kremlin spokesman said it was a "lie".
Mr Johnson warned Mr Putin that invading Ukraine would lead to Western sanctions and more Nato troops on Russia's borders.
He also tried to deter Russian military action by telling Mr Putin that Ukraine would not join Nato "for the foreseeable future".
But Mr Johnson said: "He threatened me at one point, and he said, 'Boris, I don't want to hurt you but, with a missile, it would only take a minute' or something like that. Jolly.
"But 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."
President Putin had been "very familiar" during the "most extraordinary call", Mr Johnson said.
No reference to the exchange appeared in accounts of the call given by both Downing Street and the Kremlin.
It is impossible to know if Mr Putin's threat was genuine.
However, given previous Russian attacks on the UK - most recently in Salisbury in 2018 - any threat from the Russian leader, however lightly delivered, is probably one Mr Johnson would have had no choice but to take seriously.
In his response, Mr Putin's spokesman said the former prime minister's claim was "either a deliberate falsehood, in which case you need to ask Mr Johnson why he lied, or it was not a deliberate lie. That is, he didn't understand what President Putin was saying to him".
"There were no threats to use missiles," Dmitry Peskov told the BBC.
Nine days after Mr Johnson's conversation with President Putin, on 11 February, Defence Secretary Ben Wallace flew to Moscow to meet his Russian counterpart, Sergei Shoigu.
The BBC documentary Putin Vs the West reveals Mr Wallace left with assurances that Russia would not invade Ukraine, but he said both sides knew it was a lie.
He described it as a "demonstration of bullying or strength, which is: I'm going to lie to you, you know I'm lying and I know you know I'm lying and I'm still going to lie to you.
"I think it was about saying 'I'm powerful'," Mr Wallace said.
He said the "fairly chilling, but direct lie" had confirmed his belief that Russia would invade.
As he left the meeting, he said Gen Valery Gerasimov - Russia's chief of general staff - told him "never again will we be humiliated".
Another significant encounter in the months leading up to Russia's invasion of Ukraine was with CIA director William Burns, who landed in Moscow on 2 November 2021.
Mr Burns had been circling the Russian capital for hours, as heavy fog prevented his landing, but when he finally arrived at the Kremlin he discovered Mr Putin was not there. Instead, he was sheltering in the southern Russian city of Sochi amid a spike in Covid infections.
The pair spoke over the phone.
The CIA director said he was direct in laying out the message President Biden had sent him to deliver: the US knew what Mr Putin was up to and he would pay a heavy price if he launched such an invasion.
He said the Russian president did not deny planning was underway and listed grievances about Ukraine and the West.
"I was troubled before I arrived in Moscow. And I was even more troubled after I left," Mr Burns added.
Less than a fortnight after the UK defence secretary left Moscow, as tanks rolled over the border on 24 February, Mr Johnson received a phone call in the middle of the night from President Zelensky.
"Zelensky's very, very calm," Mr Johnson recalled. "But, he tells me, you know, they're attacking everywhere."
Mr Johnson says he offered to help move the president to safety.
"He doesn't take me up on that offer. He heroically stayed where he was."