By Stephen Robb BBC News entertainment reporter

Singer says he always wanted an unknown actor in the title role

Routh, an unknown when he won the role, told a press conference there was "much more to explore" and that he would be happy to be a part of another film.

Singer added that the superhero this time faced problems it could take this movie and "perhaps beyond" to resolve.

The film, a critical and commercial hit in the US, opens in the UK on 14 July.

There had been talk of the project in Hollywood for years, with director Tim Burton and actors Nicolas Cage and Jude Law among the people linked to it at various times.

Singer believes many failed efforts approached the Superman story as if it was being told for the first time.

We are giving respect to that which has come before us, but also doing something new Brandon Routh

But for a character that originated in comic books in 1938 and has featured in numerous television series and films - most notably the 1970s and 80s movies starring Christopher Reeve - that was "kind of a flawed pursuit", he says.

"The studio took a chance with this return story, which is different, but kept the essence of what had come before," says the director.

"Hopefully you get the best of both worlds."

Co-screenwriter Michael Dougherty adds: "Everyone knows who Superman is to different degrees. We are simply trying to continue that story."

Familiar elements

As well as the main characters having obvious shared history, deliberately familiar visual elements include the exterior of the Daily Planet newspaper building and Superman's Arctic Fortress of Solitude.

Footage of Marlon Brando from the 1978 film is even incorporated - Eva Marie Saint, who starred opposite Brando in On the Waterfront (1954) and plays Clark Kent's adoptive mother, reportedly joked: "Brando would have loved this role because he got paid and didn't even have to show up."

The late Christopher Reeve played Superman in four films

John Williams' unmistakeable theme from 1978's Superman is also a critical appropriation, says Singer.

"There is something about that music that is very stirring and very inspiring," he says.

Routh adds: "People are able to accept this film, and me, because of the nods to that film.

"We are giving respect to that which has come before us, but also doing something new."

Apart from incredible special effects advances since Superman's last big screen adventures, the most significant change is in his relationship with Lois Lane, played by Kate Bosworth.

'Emotional core'

Superman returns from five years away from Earth, having visited the remains of his home planet Krypton, to find the object of his affection with a son and engaged to be married.

"That's an obstacle stronger than Kryptonite and one that's harder to navigate through, and therein lies the emotional core," says Singer.

He says that he knew during the story's development that "these were true barriers that would take the scope of the movie, and perhaps beyond, to reconcile them".

Spacey worked with Singer on 1995's The Usual Suspects

Bosworth says her nervousness about appearing in such a big-budget movie was overcome by Singer's declaration: "I don't want to make a big film, I want to make a classic film."

And Kevin Spacey, for whom the part of arch-villain Lex Luthor was written, insists the film is a blockbuster that is "still about character and about relationships".

During Superman's five-year absence, Lois Lane has won a Pulitzer Prize for an article entitled Why the World Doesn't Need Superman.

After the character's 23-year absence from cinema screens, rave reviews and an estimated $52m (�28m) opening weekend in the US suggest the real world has missed him more.

And he could be around for some time to come, Routh hints: "At the end of this film we see that there is much more to explore.