Heath Ledger's final film could be finished using special effects, says his co-star.

The 28-year-old actor, found dead in his New York apartment last week, was filming The Imaginarium of Dr Parnassus at Battersea Power Station just before Christmas.

Director Terry Gilliam hopes to use stills and Computer Generated Imagery to complete the fantasy flick, according to actor Christopher Plummer.

He told People Magazine that Gilliam was "terribly saddened" by Ledger's death and "trying to work out at this moment how to continue on".

"Fortunately, because the film deals with magic there is a way, perhaps, of turning Heath into other people and then, using stills and I think they call it CGI," he said.

"Terry was a very good friend (of Ledger). He very wants to go on with the movie, and I can very much understand why. Because he wants to dedicate it to Heath, of course."

The cause of Ledger's death will not be known for at least a week, after medical examiners complete toxicology tests.

Authorities suspect a possible drug overdose, but nothing conclusive has been determined. Several prescription drugs were found in his apartment.