Her legs were, according to a friend, “the prettiest that ever stepped into a punt or danced a foxtrot”, and her beauty is said to have enraptured Sir Winston Churchill. Now it has been discovered that a portrait of Lady Castlerosse, great-aunt of the models Cara and Poppy Delevingne, features those celebrated limbs twice. Sir John Lavery’s 1938 oil painting The Viscountess Castlerosse, Palm Springs, depicts the socialite dangling them over a diving board, with another figure seated at the side with legs crossed.

That second pair had been thought to belong to either a Hollywood director or her brother, Edward Delevingne. However, Kenneth McConkey, emeritus professor of art history at Northumbria University and a biographer of the artist, found a photograph of Castlerosse in the Lavery family collection that, he said, proved they were hers too. “Lavery was clearly aware of the universal admiration for the famous Castlerosse limbs and secretly pays his own tribute, by painting them not once, but twice,” said McConkey.

Lavery’s painting — owned by Charles Delevingne, 71, the property developer father of Cara and Poppy — is to be sold at auction at Christie’s with a guide price of $550,000-$900,000, in its modern British art evening on March 1.

Castlerosse, born Jessie Doris Delevingne in 1900, rose to fame in the 1920s when she shared a flat with the actress Gertrude Lawrence. She was reputed to have had affairs with Churchill and his son, Randolph. The future prime minister painted two portraits of Castlerosse but only one of his wife.

The daughter of a French silk and lace importer, Doris married Valentine Browne, 6th Earl of Kenmare, in 1928 but they divorced a decade later. She moved to America, but fell on hard times. Having returned to Britain in 1942, Castlerosse died of an overdose of sleeping pills at the Dorchester Hotel in London that December.

Other lots for sale in the collection include Churchill’s 1935 painting Scene at Marrakech, which he gave to Field Marshal Montgomery in the 1950s. —Liam Kelly