When Heath Ledger's family stepped onstage at the Kodak Theatre in Hollywood to accept his posthumous Oscar, it was billed as both a moving and triumphant end to the late actor's tragically short-lived career.

There is, however, a final chapter still to be written in the Heath Ledger story. For at the time of his death, the tortured genius whose Joker lifted The Dark Knight into the premier league of modern blockbusters was working on another eerily complex film role.

That movie, which will represent Ledger's true swansong, is called The Imaginarium Of Dr Parnassus. It was directed by the British former Monty Python star Terry Gilliam, and has been billed as a "fantastic" tale about a travelling circus magician who makes a series of Faustian pacts with the devil.

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Gilliam was a third of the way through the production process when Ledger was killed by an overdose of prescription painkillers. He decided to dramatically rejig his script – and invited a selection of prominent actors to perform what remained of his role.

The resulting Dr Parnassus stars Johnny Depp, Colin Farrell and Jude Law in Ledger's place. It was independently financed, cost $20m (£14m) to make, and was finished in August last year.

Yet there remains a pressing problem. Despite its stellar cast and alluring back-story, not to mention Ledger's enduring box office appeal (The Dark Knight last week surpassed $1bn in ticket sales), America at present has no prospect of seeing Dr Parnassus in cinemas.

For reasons which are not fully clear, Gilliam's film has so far failed to secure a US distribution deal. Though negotiations with several prospective commercial partners are ongoing, it has yet to be awarded a release date.

Unless it finds a theatrical distributor willing to pay the going rate for rights to screen it in cinemas, Dr Parnassus will face the fate of the hundreds of other independent films which fail to secure a deal: it will go straight to DVD.

That would be an extraordinary fate for a movie that is already the subject of a strong buzz, and which received a glowing reference by Ledger's sister, Kate, at the Oscars.

"We've seen a little bit of footage, and I think it's going to be amazing," she insisted to a backstage press conference. "Terry Gilliam is amazing and Heath always had such enthusiasm and interest in whatever Terry was doing."

The Hollywood Reporter last week published an investigation which concluded that negotiations between the movie's production company, Davis Films, and major US buyers Lionsgate and Overture during the summer had proved fruitless.

"That has fuelled all sorts of rumours in indie circles, ranging from dissent over finances on the producers' side to outsized expectations on the part of filmmakers," the Reporter noted.

At present, Dr Parnassus has effectively been "on sale" for six months. Potential buyers may be wary of its eccentric subject matter and fantasy-driven plot, which traditionally prevent a film securing mass-market approval.

Publicity photos show a white-faced Ledger in a clown's outfit. The movie's synopsis meanwhile requires a significant suspension of disbelief: it involves a surreal contest between the 1,000-year-old Dr Parnassus (played by Christopher Plummer) and the devil (known as Mr Nick, and played by Tom Waits).

Ledger's character, Tony, is a mysterious outsider who apparently travels through a dream world. In what could be either a stroke of genius on Gilliam's part or an elaborate folly, Farrell, Law and Depp play different "transformations" of Tony's personality.

"Even with the Ledger hook, a distributor would have a hard time with marketing," mooted The Hollywood Reporter. "Gilliam has grown more experimental in recent years with such fare as Tideland and The Brothers Grimm ... [and] retailing Dr Parnassus as a Ledger film risks running a word-of-mouth problem with general audiences who are not accustomed to that kind of material."

In the UK, where Gilliam has a strong following, Mandate International was able to justify splashing out for distribution rights, and is scheduled to release the film in June. Its relatively large budget means Dr Parnassus would have to reach a wide audience in order to turn a profit.

Most experts still expect the film to get a release – and rumours suggest a deal, of sorts, may be announced imminently. But unless a major buyer makes an offer, the film could be limited to a relatively small number of screens in major US cities.

Ledger's former agent, Steve Alexander, didn't respond to requests for a comment about the film's future yesterday. The British screenwriter and film producer Allan Schiach, a former friend of the late actor who was due to work with him on an adaptation of the novel The Queen's Gambit, said he thought it "inconceivable" that Ledger's swansong would never see the light of day.

"You just don't know what's going on. The producers could be asking for too much money; the distributors could want to market the film, or structure a deal in a particular way. There's all sorts of reasons a sale could take so long.

"Terry Gilliam has done this brilliant job of replacing him with these amazing stars. Jude Law, Colin Farrell and Johnny Depp are an incredible line-up, and I simply do not believe the film is unreleasable."

Bad timing: Actors who died during production

*James Dean (died 30 September 1955) Giant. The first actor nominated for a posthumous Academy Award for best actor and the only person to be nominated after death twice in acting categories, one of which was for Giant. Died in a head-on collision driving to a sports car race. The last scene he filmed of Giant was re-recorded because he died before the editing process began.

*Richard Harris (died 25 October 2002) Harry Potter And The Chamber Of Secrets. Won a Golden Globe in 1968 for his performance as King Arthur in Camelot. He died of Hodgkin's disease two-and-a-half weeks before Chamber Of Secrets premiered in the US. The year before, he said he feared his part in the Harry Potter films would outshine the rest of his long career. Michael Gambon took on his role.

*Oliver Reed (died 2 May 1999) Gladiator.

Reed died aged 61 from a sudden heart attack during a filming break from Gladiator, in which he played Proximo. It was almost exactly a year before the film's release. Due to the timing, some scenes had to be completed using computer-generated images.

Miranda Bryant