Kardashian's ex Scott Disick — who she split from in 2015 after nine years of dating — is "not happy about the engagement," a source tells PEOPLE exclusively.
"Scott never approved of Kourtney dating Travis. He has struggled with their relationship," the insider says. "He always had this idea that he and Kourtney would eventually get back together. He was pretty shocked when he found out that she was dating Travis."
Barker, 45, proposed to Kardashian, 42, on Sunday at the Rosewood Miramar Beach Hotel in Montecito, Calif. She announced the news on Instagram, sharing photos from the romantic moment with the caption: "Forever @travisbarker."
The sunset, beachside proposal "was a complete surprise" for the Poosh founder, a family friend told PEOPLE.
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"Travis told her family that he planned to propose, but everyone was great at keeping it secret," the insider said. "The Rosewood is their favorite for short weekend getaways. They often stay at the hotel. This is why Travis proposed there. It's a very special place to them, but it was also easier to keep everything a secret by proposing there."
Kardashian, who shares three children with Disick, is "ecstatic" to be engaged. "She can't wait to marry Travis. She would also love to have a baby with him," the source added.
The newly engaged couple received congratulatory messages from family and friends across social media, even from Barker's own children with ex-wife Shanna Moakler. While Moakler, 46, seemingly reacted to the news with cryptic Instagram Story posts, Disick, 38, has yet to comment publicly.
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PEOPLE broke the news of Kardashian and Barker's relationship in January. As their romance progressed, a source said Disick was "struggling" due to how serious the pair had become.
During the two-part Keeping Up with the Kardashians reunion in June, Kardashian and Disick gave each other their blessings to move on with other people. But Disick appeared to mock her relationship with Barker in August, per alleged DMs leaked by her ex Younes Bendjima.
Presley Ann/Getty; Dimitrios Kambouris/Getty
"Yo is this chick ok!???? Broooo like what is this," Disick allegedly wrote to Bendjima, sharing a photo of Kardashian and Barker kissing. "In the middle of Italy."
Bendjima, 28, allegedly responded, "Doesn't matter to me as long as she's happy. P.S. I ain't your bro."