John Lennon pulled no punches about how much he detested the Beatles' 1964 knockout photo shoot with Muhammad Ali ... that's according to the guy who snapped those iconic images.
Famed Beatles photog Harry Benson spilled the tea on the tension between the legends while celebrating 60 years of snapping pics of the band, telling Fox News Digital ... Ali wasted no time throwing shade, telling the singers their music wasn't all that.
Not only did he hit the Beatles where it hurt by questioning their musical chops, but Ali also went on to size them up, calling them "tiny, small, little men" compared to his towering frame.
Harry says Ali continued delivering knockout blows to the Fab Four's egos ... claiming he was better looking than them -- but he said that to everyone -- and was supremely confident that John and co. wouldn't sass him back, 'cause he was the champ.
It was only when the shoot wrapped the Beatles popped off ... with Lennon calling the whole thing a mistake and accusing Ali of making them look stupid.
While Benson says his vision for the shoot came to life, and he was happy with it ... he says the Beatles, including surviving members Paul McCartney and Ringo Starr, have never forgiven him.
So much for letting it be!