Spears -- who performed with Jackson at a 2001 party celebrating the 30th anniversary of his career -- was planning on catching one of the King of Pop's comeback next concerts next month in London
"I was so excited to see his show in London. We were going to be on tour in Europe at the same time and I was going to fly in to see him," she tells Usmagazine.com. "He has been an inspiration throughout my entire life and I'm devastated he's gone!"
Timberlake, who was so influenced by Jackson that he wore a fedora and glove and showed off Jackson-esque dance moves in his "Cry Me a River" video, also is mourning.
"I can't find the words right now to express how deeply saddened I am by Michaels passing," he wrote on his Web site. "We have lost a genius and a true ambassador of not only Pop music but of all music."
Timberlake continued, "He has been an inspiration to multiple generations and I will always cherish the moments I shared with him on stage and all of the things I learned about music from him and the time we spent together. My heart goes out to his family and loved ones."