The brutal realism of his work has made him the world’s most expensive living artist, with each new painting that surfaces attracting great interest from collectors.

Today six of Lucian Freud’s previously unseen paintings and drawings, including a portrait of the poet Sir Stephen Spender, are revealed in The Times. The works, from the 1940s and 1950s, had been hidden away in private collections across the world until an art historian and one of Freud’s assistants tracked them down for an exhibition in London this autumn.

Freud was 17 years old when he painted the portrait of Spender in 1940 - not that the skill with which he caught him on canvas reveals the tenderness of his age. Spender, last of the generation of 1930s English poets, was 31 at the time and, despite the gap in age, the two men were great friends, affording Freud an advantage that he appreciated. “If you don’t know them, it can only be like a travel book,” he said a year after painting Spender.

The two men spent time together at Freud’s rented cottage in Snowdonia. Spender already had a considerable reputation and was also thinking of becoming a painter. He wrote to a friend that: “He [Freud] paints all day and I write. Lucian is the most intelligent person I have met since I first knew Auden at Oxford, I think. He looks like Harpo Marx and is amazingly talented, and also wise, I think.”

The new works are among 32 paintings and drawings, dating from 1940 to 1958, exhibited together at the Hazlitt Holland-Hibbert gallery from October 9. The show has been curated with Freud’s support by Catherine Lampert, a leading academic who has staged Freud exhibitions at the Hayward and Whitechapel galleries, and David Dawson, the artist’s assistant and model for the past 15 years.

Other previously unseen images include a sketched Head of a Poet (David Gascoygne), 1945. A particularly striking oil,Portrait of a Man, c1954, depicts Brian Dean Paul, a baronet with bohemian tendencies. Every detail, down to the strands of his bouffant grey hair and his opium pallor, are captured on canvas, and Freud is said to be particularly keen on the work. “You feel the vulnerability of this man in it,” Professor Lampert said. Another painting, titled Girl in a Blanket, 1952, shows Freud’s model Henrietta Moraes, a socialite notorious for drug taking and hard drinking.

Professor Lampert said that they had aborted the search for one known portrait, having concluded that its owner, a New Yorker, had destroyed it years ago because he disliked Freud’s depiction of his double chin. How he must be kicking himself now. Prices paid by early Freud collectors pale into insignificance against today’s values. In May the Russian billionaire Roman Abramovich could not resist Benefits Supervisor Sleeping, a nude depiction of Sue Tilley, a 20-stone civil servant, which sold for a record $33.6 million (£17.2 million).

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