The work, written in 1938 when Williams, who died in 1983, was 26, has never been seen or even read until now.
After last night's breathtaking and coruscating production it is clear that his theatrical reputation will have to be re-evaluated.
For he must now be classed a social dramatist. This remarkable play was no piece of juvenilia, nor just a forerunner of his emotionally charged classics A Streetcar Named Desire and Cat On A Hot Tin Roof.
This was a searing social drama, based on a real prison riot, dealing graphically with inhumanity, cruelty, suffering, betrayal and tenderness.
Williams never wrote anything with such direct social impact again, and one can speculate now that the reason it was never produced or published is because its harrowing portrayal of prison conditions would have caused political chaos in America.
Amazingly contemporary in feel, it even brought to mind the recent Hollywood prison movie The Shawshank Redemption. But this was Tennessee Williams and of course it had many layers - memories aroused by music, lyrical and poetic language and characters who will yet become parts that every actor yearns to play.
The cold, ruthless, lecherous warden played memorably by Corin Redgrave is one. The self-taught prison trustee brilliantly and movingly portrayed by Finbar Lynch, as walls within him break down and he falls in love with the warden's secretary played by Sherri Parker Lee.
There is tangible sexual tension there, but perhaps the most vivid image of the night is the punishment scene with prisoners losing their sanity and in some cases their lives locked in a Turkish bath. A vivid and disturbing image that has been denied to us for 60 years.
Trevor Nunn, who directed last night's performance, said he had found the work "harrowing". It has, he said, "elements of the style which you would associate with Tennessee Williams, but is not really like a Tennessee Williams play. It is about a young prisoner who has a poetic sense of himself and what he wants to achieve."
The play, which emerges as an impassioned plea for justice and humanity, was sparked off by a newspaper report of a prison riot in the Thirties in which four convicts were brutally murdered.
Enormous credit must go to Vanessa Redgrave who spent a year and a half tracking down this play. She had come across a reference to the play in a new biography about Williams and went to New York to persuade his long- time friend Maria St Juste, who managed his estate, to release the manuscript.
Redgrave said after last night's performance: "It's so contemporary, so powerful. It draws in on you. And it's still happening now."
She has indeed discovered a theatrical treasure.