A unique joint work on a single sheet of paper, a handwritten poem and original drawing by AA Milne and EH Shepard, the partnership that would go on to create some of the best loved children’s books of all time, in the Winnie the Pooh series, is to be auctioned in London, with an estimated sale price of up to £50,000.

The teddy bear lying on the bed in Shepard’s drawing was the real Christopher Robin’s toy, Edward, the inspiration for the stories about the bear of very little brain and his friends in the Hundred Acre Wood.

The full manuscript, comprising poem and drawings. Photograph: Handout

The kneeling boy was Christopher himself, and Vespers, the sentimental poem the drawing illustrates, published in the 1924 volume of poems When We Were Very Young, haunted him for the rest of his life. Christopher later described the verse and the endlessly parodied lines “Hush! Hush! Whisper who dares! Christopher Robin is saying his prayers”, as “a poem which has brought me toe-curling, fist-clenching, lip-biting embarrassment”.

James Richards, a specialist on the period at Christie’s auction house, where the work will be sold on 15 July, thinks the work is enchanting, but added: “When I was brought up being read the poems and the Pooh stories, I did not realise that the real Christopher Robin did not have a good relationship with his father.”

He believes the drawing, probably originally a working drawing for the publishers showing how the poem would be set out, is probably Shepard’s earliest version of the bear that would become immortal, and the most extensive joint manuscript work by the two men.

The market for such nostalgia is avid: in December, the original drawing of the boy and bear playing Pooh Sticks set a new world record when it sold at Sotheby’s for more than £300,000, more than double the highest estimate.

Milne himself insisted that the poem was not sentimental and was even less pious: “The truth us that prayer means nothing to a child of three, whose thoughts are engaged with other more exciting matters.”