The Sun reports that Russian President Vladimir Putin was so close to dropping a nuclear bomb in October 2022 on Ukraine that crisis meetings were held in Britain about the fallout.
Former Prime Minister Liz Truss spent her final days in office studying weather maps and preparing for the possibility of nuclear bomb radiation in the UK, as US spies feared the Kremlin tyrant was hours away from being crushed of the button.
Disaster planners in Whitehall feared radioactive material released into the atmosphere could travel 1,700 miles around each blast zone across Europe.
Based on "thin" intelligence, the US concluded that there was a 50% chance that Russia could deploy a tactical nuclear weapon on Ukrainian battlefields or test a larger bomb over the Black Sea.
The horrifying details of how close the war came to a massive escalation are revealed in Out of the Blue, an updated biography of the short-lived prime minister.
It reported that Mrs Truss spent "many hours studying satellite weather data and wind directions" because of fears that "faulty weather patterns" could have a "direct effect of radioactivity passing through Britain".
Separately, a new book, War, by Watergate reporter Bob Woodward, also reports that the White House believed there was a 50% chance Russia would use a battlefield weapon based on "thin" human intelligence obtained in fall of 2022.
Then UK Defense Secretary Ben Wallace went to Washington on October 18 of that year to discuss the crisis.
Security sources told The Sun at the time that the threat had increased dramatically.
That month, President Joe Biden said that for the first time since the 1962 Cuban Missile Crisis, there was a "direct threat" of Russia deploying nuclear weapons "if, in fact, things continue the way they are going." .
The revelation comes as ministers try to play down fears that Donald Trump would abandon Ukraine when he returns to the White House in January.
Defense Secretary John Healey said he expected the president-elect to support Ukraine "for as long as it takes to prevail."
And speaking in Paris yesterday, Prime Minister Sir Keir Starmer said Britain would "support President Zelensky for as long as it takes".
But Trump's son, Don Junior, raised fears about Ukraine by sharing a video mocking President Zelensky. He scoffed as he said that "help" for Zelensky was about to end.
President-elect Trump was reported to have spoken to Putin the next day, warning him not to escalate the 32-month-old conflict.
But a Kremlin spokesman called the reports "hoaxes".