The pair - who played lovers in 2006 romantic comedy The Holiday - were apparently caught canoodling at Los Angeles' exclusive Chateau Marmont hotel.
According to an onlooker, Cameron - who has previously dated Justin Timberlake and P Diddy - was openly kissing 39-year-old Jude on the patio before they snuck off to a nearby bungalow.
The source told In Touch magazine: "When someone said, 'Get a room' they listened."
Refusing to read scripts
Mutual friend Gwyneth Paltrow is believed to have tried to set the duo up last year when she took Cameron to watch Jude in Anna Christie at London's Donmar Warehouse theatre and it appears her work has finally paid off.
Meanwhile 39-year-old Cameron is said to be refusing to read for film roles in which the character is over 40 - despite the fact she reaches the milestone in August.
An insider said: "She won't read scripts if the character is 40. Her team asks that the age be changed in the script for Cameron to read for the part."
However, the Hollywood star has previously insisted she doesn't mind getting older.
She told Cosmopolitan magazine: "I think you get better as you age. You know what you want, and you become stronger mentally, emotionally and physically."
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