Actress Sigourney Weaver has pulled out of a major environmental event in which she was expected to be one of the keynote speakers, forcing the organisers to cancel the event and consider legal actions against Weaver for "breach of contract."

Weaver informed the event organiser about a week ago via an email that she will not make it to the event and she was disappointed, Sydney Morning Herald reports.

The event, a sustainable business conference to be held in Melbourne in April, sold tickets that cost up to $995-per-head. The conference was called, "The Corporations, People & the Planet" and was going to be held at the Melbourne Exhibition centre.

Event organizer The Thin Blue Line, a company owned by Ben Brazil, was unable to find a replacement a week after Ms Weaver expressed that she would be unavailable on the duration of the event.

''Sigourney Weaver has pulled out with five weeks to go and essentially has derailed our business model for corporate tickets in the run-up to the event,'' Brazil said.

Brazil said the conference his company organised was ''a once in a lifetime opportunity to see Sigourney Weaver, Bob Geldof, Dr David Suzuki and Prof L. Hunter Lovins in one day of keynote presentations, case studies and discussion exploring a sustainable future.''

All tickets would be refunded, Brazil told the Herald.

Tickets cost $175 for students, but a group of ten could pay $9500 for a Diamond Table, which includes VIP entry to a cocktail party attended by the keynote speakers.

Weaver, who is known for her film roles in Alien, Ghostbusters and Avatar, became a renowned speaker on environmental issues after her involvement in Gorillas in the Mist: The Story of Dian Fossey.