The 20-year-old, who has appeared in RockNRolla and Sweeney Todd, told the BBC news website that taking part in the vampire series was "very exciting".
"You're the first people I've told" he said. "Shooting has already started, and I'm going over in June."
The film, due for release in November, is the second adaptation from author Stephenie Meyer's Twilight series.
The role of Caius is key to the plot of New Moon. He forms part of the triumvirate leading the Italian based coven of vampires called the Volturi.
The second book in the series sees the gang become a threat to lovers Bella and Edward.
Campbell-Bower confessed he was not previously aware of the character: "I hadn't read the books before. But it's a great thing to be a part of, and it did very well last year."
Film-maker Chris Weitz (About a Boy, The Golden Compass) has taken over directing duties from Twilight's Catherine Hardwicke for the second film.
Casting news for New Moon has been revealed gradually, with 15-year-old Dakota Fanning now confirmed to take the part of Jane, a member of the Volturi gang with psychic abilities.
Five American Indian actors will make their film acting debuts as members of a wolf pack which features heavily in New Moon.
Casting for other roles is being hotly debated on fan websites.
Campbell-Bower teased: "I do know who else will be in it - but I can't tell you, unfortunately.
"But go online and have a look at the rumours," he said, then added: "They're all false. You're putting me in a terrible situation."
The up-and-coming star will be acting alongside fellow Brit Robert Pattinson, who returns as the hero, teenage vampire Edward Cullen. Kristen Stewart reprises her role as Edward's human girlfriend, Bella Swan.
The quartet of books by Stephenie Meyer is a best-seller worldwide.
The first film adaptation, Twilight, topped the US box office, making more money than industry insiders had predicted.
Global receipts for the film currently stand at $376m (£265m).
Hundreds of fans turned out for the London premiere in Leicester Square last year, while Pattinson has become a heartthrob overnight.
Campbell-Bower said: "I doubt I'll get the same sort of adulation as Robert - but it's always nice when people recognise you for your work."
The actor also plays alongside Sir Ian McKellan and Jim Caviezel in the remake of cult series The Prisoner which is due to screen on American television this year.