Most fashion documentaries are pretty sycophantic. Not this one. The long-delayed release of Olivier Meyrou’s Yves Saint Laurent: The Last Collections – alternatively (and ironically) titled Celebration – gives us an amazingly candid and rather shocking study of the legendary fashion designer, and his apparent physical and mental deterioration at the age of 60, as he was preparing his final show in 1999 as an independent designer in the old style, before selling the YSL brand to Gucci.

The film was originally shown in 2007, but then withdrawn under legal pressure from Saint Laurent’s business and personal partner Pierre Bergé, who emerges from the film as callous, cynical, manipulative and cruel, calling Saint Laurent a “sleepwalker” who depends on his “anxieties”. Bergé is shown all but mocking what appears to be Saint Laurent’s incipient dementia as he appears not to recognise his niece at a reception.

As for Saint Laurent himself, his strange tics, tremors and mannerisms appear to be those of a very old – and overweight – man, or someone desperately addicted to medication. (An unsettling musical soundtrack underlines this.) Repeatedly, he is shown with a length of ash drooping from his cigarette, or with his head nodding as if about to black out.

He is clearly being psychologically propped up by Bergé, and yet, in some mysterious, unconscious way, Saint Laurent does appear to be imposing his diffident, desiccated personality on his studio and his kowtowing employees, like a latter-day Sun King on the Versailles court. In interviews, he is lucid and even interestingly amusing and insightful about himself: and yet in the public space he is simply not all there.

This documentary was said to be an influence on Paul Thomas Anderson’s Phantom Thread, and yet Daniel Day-Lewis’s fierce and fastidious Woodcock is more inspired by Bergé, not Saint Laurent. As for the man himself, he appears poignantly to come alive in the affectionate company of the model Laetitia Casta.

• Yves Saint Laurent: The Last Collections is released in the UK and on Mubi on 1 November.