A letter sent from counterculture figure Neal Cassady to beat writer Jack Kerouac is up for auction at Christie's on June 16.

It's known as the Joan Anderson letter and is famous for inspiring Kerouac to adopt his trademark stream of consciousness style of writing.

This letter from Neal Cassady inspired Jack Kerouac's writing style

It runs to 16,000 words over 18 pages and was written in the grip of a Benzedrine high.

Kerouac explained in a 1968 interview with the Paris Review: "I got the idea for the spontaneous style of On the Road from seeing how good old Neal Cassady wrote his letters to me, all first person, fast, mad, confessional, completely serious, all detailed…

"The letter, the main letter I mean, was forty thousand words long, mind you, a whole short novel.

"It was the greatest piece of writing I ever saw, better'n anybody in America, or at least enough to make Melville, Twain, Dreiser, Wolfe, I dunno who, spin in their graves…Neal and I called it, for convenience, the Joan Anderson Letter."

The letter is so named for its references to Cassady's then girlfriend, Joan Anderson.

Cassady was a key figure in the American counterculture of the 1950s and 1960s.

He was an outsider raised on the streets of Denver who became friends with heavyweight intellectuals such  Kerouac and Allan Ginsberg.

Kerouac went on to immortalise him as the mercurial, puckish figure of Dean Moriarty in On the Road.

Cassady wrote extensively, but very little of his output has survived. He is perhaps best remembered for his ability to ingest titanic quantities of drugs.

The letter, long believed lost, is expected to sell for around $400,000-600,000.

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