British actor William Moseley and actress Laura Brent pose for photographers as they attend the World Royal Premiere of the new film 'The Chronicles of Narnia: The Voyage of the Dawn Treader' in Leicester Square, London November 30, [Photo/Agencies]

LONDON  – The makers of the latest blockbuster based on C.S. Lewis's "The Chronicles of Narnia" books admit they departed significantly from the source material, but believe the changes made for a better film.

"The Chronicles of Narnia: The Voyage of the Dawn Treader" hits movie theatres this week in the key U.S. and British markets, and much is riding on the third installment of the franchise.

The action-packed, effects-laden 3-D film follows Lucy and Edmund Pevensie and their surly cousin Eustace to Narnia where they embark on an epic sea adventure with Caspian and the feisty talking mouse Reepicheep.

British director Michael Apted, best known for Bond film "The World is Not Enough," said the Lewis estate initially questioned the need for changes to the storyline made in the script, but eventually agreed that they were necessary.

The original Lewis novel, third in the seven-book children's series, was too episodic for the big screen and lacked a narrative thread to drive the plot forward, Apted said.

"You've got to have a reason in a movie to go from A to B to C, especially in a commercial movie. So that was a big problem," Apted told Reuters in a recent interview.

He and the screenwriters spent two years coming up with an alternative, eventually deciding to borrow from the fourth Narnia novel "The Silver Chair" and its theme of captives being held underground as a way of moving the story along.

"They (the Lewis estate) were a bit put out by it at first," Apted added. "We gave them the original script to look at which was the pure adaptation of the book, and I think that they could see that there was just an inner inertness to the story."

REVIEWS MIXED

By using elements of The Silver Chair, production company Walden Media and new partner Twentieth Century Fox may find it easier to make the fourth movie in a franchise which Apted argued would be difficult to sustain for seven titles.

"I would be surprised if they did all seven books," Apted said. "I suppose the fourth one might work better now after the changes we made in the third ... but after that it goes haywire.

"With a franchise, the continuity of characters is actually quite crucial and I don't know how they are going get beyond book four. I think it is a problem."

He said he would be prepared to direct a fourth film were it to be made and if he was asked.

Mark Johnson, a producer on all three Narnia movies, told a press conference last week he would like to adapt all seven C.S. Lewis novels.

Early reviews of The Dawn Treader have been generally positive, with review website www.rottentomatoes.com giving the movie an aggregate score of 61 percent.

"It's thoroughly satisfying and enjoyable and leaves you wanting more ... roll on (hopefully) the next in the series, The Silver Chair," wrote Mark Adams in the popular British tabloid newspaper The Mirror.

But The Hollywood Reporter's Todd McCarthy was less impressed, saying: "This tedious third entry doesn't bode well for the continuation of the franchise."

Walt Disney Co. partnered Walden Media in the first two films before dropping the franchise.

"The Lion, the Witch and the Wardrobe" was a hit with global ticket sales of $745 million, but sequel "Prince Caspian" was seen as a failure despite earning $420 million in ticket sales.