A fourth movie in the "Back to the Future" series looks unlikely, one of the films' executive producers said on Monday, as fans of the time-travelling comedies gear up this week to mark the exact day that Marty McFly travelled to from 1985.

With this year marking the 30th anniversary of the first blockbuster film, some fans had hoped for a revival of the franchise at a time when others movies are getting reboots.

Christopher Lloyd, who played Dr Emmett "Doc" Brown in the trilogy, told the Hollywood Reporter last week he would reprise the role if the whole cast, including Michael J. Fox, who played McFly, and director Robert Zemeckis returned.

Fox, now 54, was diagnosed with Parkinson's disease in 1991. He semi-retired from acting in 2000 as his symptoms worsened and focused his energy on research for a cure. But he has since done a few television shows.

"You have to be careful when you're doing a sequel or a 'prequel', because it's really about the story. You can't just do the same story over and over again," producer Frank Marshall told Reuters.

"I think it's OK to have sequels, but they have to be their own movie as well ... I'm happy to see things like "Jurassic", like "Bourne". I don't think we're going to see another "Back to the Future" ... I think that one's done."

This Wednesday, Oct. 21, is being dubbed "Back to the Future Day". McFly and Doc travel there in the second film to find a world of flying cars and hovering skateboards or 'hoverboards', along with phenomena that turned out to be closer to the mark such as wearable eyeglass-screens and video calling.

Marshall, whose credits also include "Indiana Jones" films and the "Bourne" series, was speaking at an event marking the DVD release in London of the summer blockbuster "Jurassic World", to which a sequel has been confirmed for 2018.