Penn Badgley and Blake Lively, both 23, have ended their three-year romance, a rep for the Gossip Girl costars exclusively tells the new Us Weekly (on newsstands now).

They called it quits in mid-September.

"They’re professionals," an insider tells Us Weekly. "They're still good friends and hang out on the set."

The source says that the duo -- who met as preteen actors in L.A. -- "kept this news to themselves for a while."

Indeed, they have always taken a drama-free approach to their relationship.

Badgley told Us last fall that working together while dating didn't present any problems: "Some would say it’s a double-edged sword, but in our case, we manage to balance it well."

Even those closest to the twosome may be shocked to hear the news.

"Nobody on set had any idea," says the insider. "They're really just private."