The late director's verified social media accounts confirmed Vitali's death Sunday ... writing, "It is with the greatest of sadness that we have to tell you that the mainstay of a vast number of Kubrick's films, Leon Vitali, passed away peacefully last night.  Our thoughts are with his family and all that new and loved him." No other details were offered.

Kubrick's account goes on to heap on the praise for Vitali ... noting his behind-the-scenes work with his film master, but also pointing to the roles he portrayed on camera as an actor -- including as one of the leads for SK's 1975 period drama "Barry Lyndon."

The post also notes this ... "Whether giving the performance of his life as Lord Bullingdon, portraying the ominous Red Cloak, providing BTS technical support on FMJ and attending the Cannes Film Festival 4K release of The Shining, Leon Vitali was the heartbeat of Kubrick’s films after the master himself." So yeah, he's very important in the Kubrick-universe.

As has been well-documented, the guy was hopelessly devoted to Stanley ... taking on the challenging role of becoming his effective right hand and enforcer on film sets, including for flicks like 'The Shining,' 'Full Metal Jacket' and Kubrick's final movie, 'Eyes Wide Shut.'

He's credited in each film as "Assistant to Director," and was even the CD in the latter 2 films. But Vitali's involvement in these films went well beyond just his titles -- he was literally running around on set alongside Kubrick and making sure his boss's vision came to life.

Kubrick, of course, was notorious for his grueling shoots ... doing dozens of takes of any given scene and demanding absolute perfection from his cast and crew -- which applied to Vitali as well. Still, he hung tight and stayed loyal throughout the years until SK's own death.

A lot of Vitali's journey with Kubrick was chronicled in the 2017 documentary 'Filmworker,' which cleaned up at a ton of festivals upon its release ... giving great BTS insight into the life and work of an expert artist, through the lens of LV.

As many of the actors who worked on Kubrick sets point out ... Vitali was involved in just about every step of the production process, oftentimes serving as an acting coach for iconic characters and scenes.

He wasn't just behind the camera, though -- Vitali has a fair amount of acting credits to his name ... including in films such as 2013's 'Romeo and Juliet,' 'Little Children,' 'Inter Rail,' 'Terror of Frankenstein' and a ton of TV series and sitcoms from the '70s.

Vitali even played Red Cloak in 'EWS' -- the leader of the sex-crazed cult that Tom Cruise infiltrates. He's survived by his wife and his children ... Vitali was 74.