The 32-year-old socialite and Jackson, 37, called it quits about 2 weeks ago, a source tells ET. The couple dated for three years, with rumors of an engagement were circulating in 2013.
The source also reveals that Middleton spent the night at the house of her ex-boyfriend, James Matthews, last week.
This wasn't the only time the Duchess of Cambridge's sister has been seen with Matthews recently. She spent time with him and her parents, Carole and Michael Middleton, in St. Barts at Matthews' family's luxury resort, Eden Roc, in August. Despite their quality time together, the source also says that two are not getting back together.