Sean Penn and Charlize Theron split - report

Sean Penn and Charlize Theron have reportedly split after 18 months of dating.

The Oscar-winning couple struck up an unlikely romance in late 2013, but now multiple sources tell Us Weekly that Theron, 39, has ended the relationship.

An insider claims the Monster actress called time on the romance during the couple's recent visit to the Cannes Film Festival in France last month (May15).

Although Theron and Penn, 54, have known year other for almost two decades, the friendship-turned-romance took the actress by surprise, and in a recent interview with Vogue, she said, "I really didn’t see it coming. Sean and I have known each other for 18 years. We were just really good friends. It was effortless and kind of naturally happened, and before I knew it, I was in something that was making my life better."

In December (14), the couple sparked rumors of an engagement, but neither confirmed the reports.

Both actors have had high-profile relationships in the past - South African beauty Theron has dated Irish star Stuart Townsend and Third Eye Blind frontman Stephan Jenkins, while Penn was married to Madonna and Robin Wright.

Representatives for both Theron and Penn have yet to comment on reports of a split.