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Power & Beauty: The Art of Sir Oswald Birley, to run until October 10, was curated by art historian Dr Jonathan Black and Josephine Fitzalan Howard.

Mould said: “We have raided the stately homes of Britain to find 40 of the portrait painter’s best works – including the Churchill bought by Lloyd Webber for £1.6m.”

It is the first major exhibition of the artist's work since the 1954 retrospective that followed his death in 1952. Mould said it will provide a “rare opportunity to restore one of the most sought-after artists of his day to his rightful position within the canon of 20th-century British figurative art”.

From 1919 until the early 1950s Birley was one of the go-to artist of the time, commissioned by the Royal family, the aristocracy, politicians, artists and scientists.