Pamela and Randolph Churchill (left), Pamela and Averell Harriman (right) (Image: GETTY)
Mr Roberts had exclusive access to the love letters of Pamela Harriman, which reveal the affairs, whilst researching for his new biography Churchill: Walking with Destiny.
He said: “At one point three delegates to the Yalta Conference were writing her love letters!
“The new source allows us glimpses into the Churchill during the war because all the men she was sleeping with worked with Churchill, and wrote to her about him.
“She slept platonically in the same bunk as Churchill during air raids, because his wife Clementine didn’t like his snoring!”
One of the men she was romantically involved with was American politician W Averell Harriman, who would later become her third husband.
Mr Harriman was President Roosevelt’s special envoy to Europe during the war and served as ambassador to the Soviet Union for a time.
Pamela was also involved with the famous American journalist