As Radar revealed, Affleck, 44, recently was caught canoodling with Saturday Night Live producer Lindsay Shookus in London.
Affleck met Shookus, 37, while hosting SNL, and insiders noted he spent some serious time flirting with her, even though he was still married at the time to Garner.
"There was an immediate spark," a source told Radar. "Ben and Lindsay flirted and stayed in touch through Ben's appearances on SNL."
After Garner and Affleck separated, he reached out to Shookus, and their relationship took off. And while Affleck put on a show in public of trying to reconcile with his estranged wife, in private, he was spending time with Shookus, showering her with pricey gifts and lots of affection.
Garner found out about the fledgling relationship when a swanky jewelry store called their house about a ring Affleck bought for his new gal pal.
That's when Garner decided to confront Affleck's mistress.
"Jen met face-to-face with Lindsay and demanded she return the jewelry," the source revealed. "Lindsay says she'd return it, but only if Ben asked for it back."
Before Garner's discovery, there was still some hope she and Affleck would settle their differences. The two were even photographed spending the July 4 holiday weekend together with their kids, Violet, 11, Seraphina, 8, and Samuel, 5.
However, Affleck has clearly made up his mind. He spent time with Shookus in London in the days before the family weekend with Garner and their kids.
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