Letters written by British artist Lucian Freud when he was a teenager to his friend, poet and critic Stephen Spender, are to be sold at auction next month.

Ten unpublished letters which have emerged from the Spender family collection after 70 years will be offered at Sotheby’s Contemporary Art Day Auction on July 2 with a combined estimate of €38,000-€57,000.

Freud (1922-2011) was perceived as being so reluctant to talk about his late teenage years that he came under criticism for constructing his own, mythical narrative of his youth.

The unseen letters date to 1939-1942 when Freud was studying under the tutelage of artist Cedric Morris at the East Anglian School of Painting and Drawing in Dedham, Essex, the UK – one of the only art schools to stay open during the war.

Freud and Spender had met when the artist was still a pupil at Bryanston School in Dorset, and Spender was thinking of becoming a teacher. Their families were well known to each other, living in the same building in north London, and despite a 13-year age gap, the two became close friends.

One of the unpublished letters written by Lucian Freud when he was a teenager to his friend, poet and critic Stephen Spender.

The relationship was to prove decisive in this crucial period of the artist’s early career. Spender was the subject of many works by Freud in 1940, mostly conceived during the month the two spent together at a retired miner’s house in Wales that the artist had rented to escape the bombing in London.

One portrait, an early oil painting, became the artist’s first published work when later that year Spender printed it in the influential Horizon magazine which he co-edited – a hugely significant opportunity for a young artist not even out of his teens.

Filled with affection, delight and the unexpected, the boyish letters reveal the wild imagination of a witty young artist at the outset of his career.

Populated by drawings and watercolour paintings, one letter shows a man pulling a miniature horse on a lead. In another, a figure balances on the head of a flying bird and a small man rides a horse atop an ear.

Addressed to ‘Spethan’, ‘Schuster’ or ‘Step-hanio’ and signed off ‘Lucelli’, ‘Lucio Fruit’ or ‘Lucionus Fruitata’, he delights in preposterous scenarios and impossible situations.

At the very heart of the collection is a letter from 1940 featuring what can only be assumed to be a self-portrait possibly based on Morris’s portrait of Freud, now in the Tate collection.

Alongside the drawing Freud writes: “Cedric has painted a portrait of me which is absolutely amazing. It is exactly like my face, is green, it is a marvellous picture.”

Oliver Barker, senior international specialist, contemporary art, said: “While relatively little is known about Freud’s teenage life, the emergence of these letters is a sensational moment, providing a glimpse into the workings of a truly artistic mind. More than just letters, they are artworks in their own right. Filled with drawings and watercolours, they show the workings of the artist, reflecting his artistic output at the time.”