KARDASHIAN clan matri­arch KRIS JENNER’s 21-year marriage is on the verge of collapse – after hubby Bruce Jenner caught her cozying up to her boytoy lover!

Momager Kris, 56, secretly re­kindled her passionate relationship with Todd Waterman – the young hunk who was her lover during her marriage to the late Robert Kar­dashian – earlier this year, sources say.

Olympic champion Bruce, 62, was blindsided when he walked in on Kris composing an e-mail to Todd, insiders say, but what he doesn’t know is that Kris also met up with her ex-lover for a hush-hush roman­tic rendezvous!

“Kris told no one, but she did go out with Todd, and it was an electrifying date,” said an insider. “Kris walked out of the restaurant where they’d met looking like the cat that ate the canary. It’s so obvi­ous that she still carries a torch for him.

“After the dinner, Bruce totally busted Kris when he caught her red-handed, e-mailing Todd. Bruce glanced at her computer screen and when he saw that Kris had typed ‘xoxo,’ he de­manded to know who she was corresponding with. When Kris reluctantly admitted it was Todd, Bruce flipped.

“Kris made lame excuses, claim­ing she needed final closure, but Bruce isn’t buying it. I think he be­lieves Kris was planning a steamy hookup with Todd and would have carried through with it if he hadn’t caught her.

“He threw a fit and stormed out of the house.”

A close source added: “When it comes to Kris, Bruce feels in his heart that ‘Once a cheater, always a cheater.’ I don’t think he can be­lieve that Kris has had the audacity to keep it up with Todd when their marriage is on the line. Cheating is a dealbreaker for him, so their mar­riage is teetering on the edge.”

Bruce has been reeling ever since his wife’s tell-all memoir, “Kris Jen­ner…And All Things Kardashian,” was published in November 2011.

In the book, Kris bared intimate details of her affair with Waterman, a former pro soccer player 10 years her junior. At the time, she was still married to Kardashian, the late O.J. Simpson attorney who’s the father of her daughters Kourtney, 33, Kim, 31, Khloe, 28, and son Robert Jr., 25.

Describing their fiery passion, Kris wrote: “Wild crazy sex all the time. We had sex in cars, sex on the tennis court, sex in the pool house, sex in the garage when we got home, sex up and down the stairs, sex everywhere, all the time.”

In a recent interview, now 46-year-old Todd admitted that he continued an intimate relationship with Kris even after she started dating Bruce – the gold medal decathlon winner at the 1976 Summer Olympics.

The two stayed in touch after Kris wed the sports hero in 1991, and she even tried to arrange a clandestine meeting with Todd in 2000.

In a recent episode of the family’s hit E! reality program “Keeping Up with the Kardashians,” Kris bumped into Todd, now a Hollywood ani­mator, at a Los Angeles-area tennis club.

She gave him a lengthy hug, exchanged contact in­formation with him and later mulled over whether to meet him again. On-air, Kris claimed she had second thoughts and bailed on the clan­destine get-together.

But Kourtney blew her mother’s cover during a family vacation in the Dominican Republic, reveal­ing that Kris and Todd had been in touch.

Stunned, Bruce splut­tered: “I am at a loss for words at this point. Why would she even make contact with this Todd? This is a bunch of bull, I’ve lost my appetite” – before stomping away from the dinner table.

Although Bruce continued to brood throughout the Dominican Republic trip, Kris eventually had a heart-to-heart with him, insist­ing that she didn’t go through with seeing Todd. Bruce forgave her for having a midlife crisis crush and said he always trusted her.

“Bruce would feel totally be­trayed if he actually knew the length and breadth of Kris and Todd’s current relationship,” the insider divulged. “They had con­tinued to feverishly e-mail each other, flirt and text.”

And THE ENQUIRER has learned that Kris did, in fact, meet up with Todd for a romantic dinner at an L.A.-area restaurant.

“I think Kris is covering up a lot of secrets and lies,” said the insider. “After the latest drama, Kris calmed Bruce down by swearing up and down that she loves him, is devoted to their family and would never do anything to jeopardize it,” said the source.

“But Bruce’s feel­ings are still raw. I don’t think he can take much more, and will be wondering what else Kris is up to behind his back.”