James Devaney / WireImage 1 / 7
James Devaney / WireImage 1 / 7

By Kat Giantis

For months, rumors have been flying over Justin Timberlake's supposed infatuation with his "Friends With Benefits" co-star, Mila Kunis. And within a day of his breakup with Jessica Biel after four years of togetherness, those rumblings started to grow louder.

"Justin is very interested in Mila," a source close to the pocket-size starlet tattles to E! Online. "They've been flirting, but so far nothing has happened."

Talk of Timberlake's ostensible interest in Kunis first began back in September, when Us Weekly claimed he was "smitten" and "absolutely obsessed" with her.

The chatter picked up steam last month when Justin, 30, appeared "cozy" with Mila, 28, during a SAG Awards afterparty and was said to be "aggressively pursuing" her.

But an insider assures People that the "Black Swan" actress, who called it quits with Macaulay Culkin in January after a low-profile, eight-year romance, played no role in Timberlake's decision to part ways with Biel.

"Mila had nothing to do with their breakup," insists the mole. "In fact, no one came between them. They decided their relationship ran its course and it was time to move on."

RELATED: Justin and Jessica's plentiful PDA shortly before going belly up

Timberlake and Kunis are "not in a relationship," continues the snitch. "They star in a movie called 'Friends With Benefits,' but they're just friends. That's it. They are not together."

Meanwhile, there are theories aplenty about what led to Justin and Jessica's split.

First up: Timberlake's purportedly wandering eye, whispers of which began in earnest back in October when Us Weekly claimed he'd been "caught cheating" with Olivia Munn.

The rumors resurfaced in January when Life & Style accused the crooner-turned-actor of sending flirty texts to Munn, including one that said, "My relationship is basically over."

RELATED: Olivia talks to Wonderwall about the Justin rumors

One source maintains to People that Biel "looked the other way" when it came to Timberlake's alleged monkey business, while another points out, "There was never any proof. When you're in a relationship, you have to choose to believe the person or get out."

The second insider asserts that the bust-up "really was mutual. There really wasn't a final straw. They're just two people who realized they wanted different things in life and were headed in different directions. … It was just easier to end it now rather than later."

Also weighing on possible reasons behind the split is the London Daily Mail, which says signs of trouble emerged during a party Jessica tossed for Justin's 30th birthday earlier this year.

"Jessica was overcome with emotion when she stood up to toast her boyfriend on reaching the milestone," recalls a source. "She quickly welled up as she reminisced about the first time she met Justin and joked about how she virtually stalked him into going on a date with her."

The paper says Biel gushed to the guests about her love for her beau and "went on at length about how much he meant to her."

Timberlake's supposed response: "Yeah, b-tches!"

"It was really awkward," relays the insider. "Justin's reaction was almost disrespectful. He just laughed it off."

And that's exactly what Biel appears to be doing now that she's flying solo.

The actress sported a big smile, a leather jacket and an oversized feather earring as she stepped out Saturday night in Los Angeles for a dinner celebrating her younger brother's birthday.

"It's never easy when a relationship ends," a spy tells People, "but she knows this needed to happen, and she's moving on with her life."

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