Downey Jr cannot fly into the UK
Iron Man 2's world premiere has been moved from London to Los Angeles because of uncertainty over air travel between the UK and US.
Stars including Robert Downey Jr, Mickey Rourke, Gwyneth Paltrow and Scarlett Johansson were all due to attend the event on 26 April.
But Paramount Pictures and Marvel Entertainment have relocated the promotional push to the the US.
The first Iron Man film took $318m (£208m) when it was released in 2008.
The sequel sees Downey Jr reprise his role as playboy millionaire-turned-superhero Tony Stark.
It is not known whether the star's scheduled appearance on this week's Jonathan Ross show will be affected by the decision to move the film's premiere.
Paltrow, who is normally based in the UK, is also due to take part.
It is hoped that a special screening of Iron Man 2 will still go ahead in London on 26 April.