The couple will have a low-key ceremony at the posh Grand-Hotel Du Cap-Ferrat in Cap-Ferrat on Saturday, according to London's News of the World.
"It is fully booked for a special event next weekend. They have booked the whole hotel," a source tells the paper. Timberlake's spokesman would not confirm the report, saying, "We never comment on anything having to do with his personal life."
When asked directly by The Post if the hunky singer was about to walk up the aisle, the spokesman, Ken Sunshine, issued a terse, "No comment."
Diaz's spokesman did not return a call.
Timberlake, 24, and Diaz, 31, fell in love at the Grand-Hotel when they stayed there during the Cannes Film Festival.
They apparently hoped to keep their upcoming nuptials a secret.
"[They] really wanted to be able to get married out of the spotlight. They knew if they did it in L.A., it would turn into a media circus, so they went abroad," a source told the newspaper.
The newspaper said it was Ozzy and Sharon Osbourne's camp that who let the cat out of the bag.
Asked why they wouldn't be watching daughter Kelly's gig in London next Saturday, they replied: "Because they'll be in Nice for J.T.'s wedding."
In other news, Timberlake will provide the voice for a new character in "Shrek 3," the latest sequel to the animated ogre franchise voiced by Diaz and Mike Myers, DreamWorks announced Sunday.
Timberlake will play a young King Arthur, or "Artie," King Harold's rebellious nephew.
Diaz voices Princess Fiona, Shrek's wife. It will be the job of Shrek (Myers) to teach Timberlake's character to act like royalty.
Timberlake appears in two upcoming films, "Edison," starring Kevin Spacey and Morgan Freeman, and "Alpha Dog," starring Bruce Willis.
"Shrek 3" is slated for release in May 2007.