Did Reese Witherspoon dump her husband because he fell in love with his beautiful co-star?

The ENQUIRER has uncovered a shocking scandal behind the breakup and learned exclusively that Reese ended her marriage after she found secret messages that husband Ryan Phillippe sent to another woman on his BlackBerry, say sources.

The ENQUIRER has also learned that Ryan became EXTREMELY close to Abbie Cornish, his beautiful Australian co-star on a recently filmed movie with the working title “Stop Loss.”

In fact, after the last day of filming in Austin, Texas, Ryan and Abbie had a cozy dinner alone at Kenichi, an Asian restaurant.

“They came into the restaurant a month ago, both casually dressed in warm-up outfits,” an eyewitness told The ENQUIRER. “They asked for the restaurant staff to pull curtains around their table, so no one could see them.” But, according to one unconfirmed posting on an internet blog, a diner could see around the partition and claimed to watch them making out.

“We sat two tables away from Phillippe and an extremely attractive blonde (not Reese),” a blogger wrote on a Web site called “Pink is the new Blog.” “The couple had a partition placed around their table soon after they arrived, but we could still see their legs intertwined under the table.

“As we left, we could see them clearly making out behind the partition” said the blogger.

Reese, 30, and Ryan, 32, have admitted to attending marriage counseling. Sources now tell The ENQUIRER that when Reese found apparent email proof of Ryan cheating it finally convinced her to end the troubled relationship.

“Reese is heartbroken,” a source told The ENQUIRER.

Reese discovered Ryan’s messages when she accompanied him to New York for the premiere of his movie “Flags of Our Fathers” on Oct. 16.

At the premiere the couple smiled and posed for photos together, “but when they got back to the hotel, Reese found Ryan’s BlackBerry, which he’d carelessly left in the bathroom — and she read it,” said the source.

“Ryan was caught red-handed. All he could say was he didn’t mean what he’d written to the other woman.”

Ryan went to Morocco to finish filming “Stop Loss” and hoped Reese would cool off while he was gone.

But when he got home to their Brentwood, CA., mansion in late October, he was no longer welcome. The marriage was over and Reese kicked him out.

Before announcing the split, Reese contacted Los Angeles celebrity divorce attorney Robert Kaufman, who has represented Jennifer Aniston and Lisa Marie Presley.

Ryan and Reese — who’s now Hollywood’s highest paid actress after finalizing a $29-million deal for her next film — will now have to determine custody of their two children, 7-year-old Ava and Deacon, 3.

A spokesperson for Abbie Cornish says that she and Ryan are costars and that is all.