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Partners
The couple - who co-starred in 2011 romantic-comedy The Oranges - only went public with their relationship in February, but are allegedly already planning to tie the knot after the 33-year-old actor secretly popped the question earlier this week.
An insider told America's Star magazine: "Leighton and Adam are crazy about each other and have talked about getting married eventually.
"But she had no idea when he would propose so soon! It was the surprise of her life and she's ecstatic!"
The 27-year-old beauty - who is known for her role as Blair Waldorf in teen drama TV series Gossip Girl - and her beau are keen to have a summer wedding, but are still undecided on the location of their nuptials.
The insider explained: "Right now their plan is to get married next summer.
"They are still deciding if they want a destination wedding or if they'll marry in southern California somewhere."
Adam was previously romantically linked to 34-year-old director Lorene Scafaria - who helmed Steve Carell and Keira Knightley's comedy drama movie Seeking a Friend for the End of the World - while Leighton has previously dated Austin Powers actor Aaron Himelstein.
