The Houses of Parliament had commissioned the British artist to paint a portrait of the wartime prime minister for his 80th birthday in 1954.

An episode of hit Netflix drama The Crown revolved around the creation of the painting and showed Sir Winston's wife Clementine watching it go up in flames following its unveiling.

The preparatory painting, which shows Sir Winston side on against a dark background, will be exhibited at Blenheim Palace in Oxfordshire before it goes under the hammer.

It will go on view to the public from April 16 to 21, displayed in the room where Sir Winston was born 150 years ago.

The oil on canvas painting will then travel to Sotheby's in New York where it will remain from May 3 to 16 before it travels to London, where it will be exhibited between May 25 to June 5.

On June 6 it will go to auction with a guide price of PS500,000 ($A970,600) to PS800,000 ($A1.6 million).

Over the course of his life Sir Winston was painted by artists including Sir William Orpen, John Singer Sargent, Walter Sickert and Sir Oswald Birley.

Andre Zlattinger, Sotheby's head of modern British and Irish art said: "The name Churchill evokes for each person a different snapshot of a multifaceted man.

"In this rare portrait, Churchill is caught in a moment of absent-minded thoughtfulness, and together with the backstory of its creation, it gives the impression of a man truly concerned with his image.

"This version shows Churchill closer to how he wished to be perceived, his less austere and gentler side, and so it is tempting to imagine how his reaction might have differed.

"Having remained within the close circles of the artist and the sitter for generations, the sale of this work is an opportunity to acquire a piece of history."

Australian Associated Press