Sure, JEN throws a million dollar B-day party for hunky BF JUSTIN but she’s warning KATIE COURIC to stay the %&*# AWAY! The FULL story — with ALL the sassy deets.

JEN lashed out at Katie Couric after becoming convinced that the 56-year-old cougar was trying to get close to her fiance, Justin Theroux. “Stay away from my man!” she warned the veteran newswoman, according to a source.

The cat-fight burst spectacularly into the open on July 31, when Jennifer, 44, was a guest on E! channel’s “Chelsea Lately” show.

Host Chelsea Handler, a close friend of Jennifer’s, told her pal that during a recent appearance on her show, perky Katie – now hosting a syndicated talk show – tried to squeeze secret information from her about Jen’s upcoming nuptials.

“Katie was asking me about your wedding, and I had to act like I didn’t even know you were engaged,” Chelsea told Jen.

Jennifer responded by saying: “What’s Katie Couric? Is she a legitimate journalist?…Wasn’t she on the ‘Today’ show?”

Handler acidly replied: “She was on the ‘Today’ show. She got a demotion to her own show.”

Chelsea’s tip-off has apparently put Jennifer on high alert.

“Jennifer is convinced that Katie will stop at nothing to get details about her wedding, which she’s kept under lock and key,” said the source.

What’s more, insiders say that Jennifer fears Katie may be interested in more than wedding plans when it comes to Justin.

“Katie clearly likes her men dark, handsome and young, and Justin fits that bill,” said the source. At 42, he’s 14 years younger than Katie, which the source says is “perfect for her.”

Katie dated Brooks Perlin, a 17-years-younger toy boy, for five years before hooking up with her six-years-younger banker beau John Molner, 50. She also dated trumpeter Chris Botti, who’s five years her junior.

Incredibly, Katie’s now telling pals she has a crush on Baltimore Ravens quarterback Joe Flacco – who’s only 28.

Revealed an insider: “Katie is just like Demi Moore – a cougar who has a thing for young studs.”

Couric previously characterized the “cougar” label as “silly and sexist.”

Jennifer’s feud with Katie comes at the happiest time in her life. Her romance with Justin is going so well she recently said that she already feels married to him. And Hollywood was abuzz about a possible pregnancy when she was spotted out and about with what looked like a baby bump.

Jennifer’s reps have denied she’s pregnant, but the source says: “Jennifer has a lot to protect right now, and the last thing she wants is for Katie to ruin her happiness.”