Whitney Port is reportedly having a "full-blown" love affair with her beau Tim Rosenman.

The 26-year-old fashion designer met her associate producer boyfriend Tim three years ago on the set of her former TV reality show The City.

Whitney was involved with her boyfriend Ben Nemtin at the time, but the pair split in June 2011.

Whitney always had a soft spot for Tim and their romance flourished after they began dating late last year.

"[They] kept in touch and have always had crushes on each other," a source told Us Weekly magazine. "Now they are full-blown together. [It's] serious."

Whitney and Tim share many of the same interests and enjoy spending time together.

The pair vacationed together in Miami in January and are often flying across the country in search of fun.

"They have mutual friends from New York and L.A.," the source continued. "His sister works in fashion, too. They run in a similar crowd and seem to be really happy."

Whitney will unveil her Fall 2012 Whitney Eve collection at New York Fashion Week next Wednesday.