Justin Timberlake and Rihanna: Reportedly On!

Justin Timberlake and Jessica Biel might be broken up - but they are definitely having relationship problems.

In the latest issue of Star, the supermarket tabloid reports on the one-word reason for the tension between Justin and Jessica: Rihanna.

Reportedly, JT brought SexyBack under RiRi's Umbrella at a MTV Video Music Awardsgathering on September 13, as the singers grinded in front of onlookers... and then moved on to her hotel room for a "steamy, private after-party," the magazine states.

“They tried to keep it a secret, but Jessica found out within days,” said a source, adding that Biel "must be a wreck" over the news.

While the publication won't come out and label Timberlake and Rihanna as an official item, it does go into detail about their close contact. You might wanna turn away (and unclench your fists), Chris Brown:

“Rihanna and Justin have been talking and texting on the phone nonstop since the VMAs,” sand ain insider. “She asked him on the phone, ‘Are you still with Jessica?’ And he hinted that things were cooling off between them.”

But it's quite the opposite for this gorgeous pair. Sources say Rihanna "wants to date" JT and even gave him an impromptu lap dance during a recording session this month!