Leto, who played the DC villain in 2016 film Suicide Squad, was said to be unhappy when Todd Phillips’s new film was announced.
According to Hollywood Reporter writer Tatiana Siegel, Leto felt "alienated" when Warner Bros greenlit the new film.
In 2018, it was announced that Leto would star in his own standalone film, which would have been billed as a Suicide Squad spin-off.
However, both that film and a Harley Quinn & Joker movie – that would have re-teamed Leto with Margot Robbie – were cut from Warner Bros' slate of DC projects.
Leto is also notably absent from the upcoming Suicide Squad sequel. When making the original film, Leto reportedly sent a dead pig to the first table read.
Joker is out now in cinemas.