The 30-year-old, who is currently a judge on 'The X Factor', is working out like crazy to shape up for their big day, reported Sun Online.
"It's very much a countdown to the nuptials right now. We're talking within weeks. She's put a treadmill into her 'X Factor' dressing room and is making sure she's jogging as often as she can.
"She's taking her look incredibly seriously and has said she will dazzle with her outfits and her body on the live finals of 'X Factor'," a source said.
The singer got engaged to Jason, her former agent, last December and the couple recently purchased a new USD 08.5 million house together.
Spears, who has two sons Sean Preston, seven, and Jayden James, six, with ex-husband Kevin Federline, has walked down the aisle twice before.
She married former childhood friend Jason Alexander in an impromptu ceremony in a Las Vegas chapel before it was annulled 55 hours later in January 2004.
Nine months later she married Kevin - a former backup dancer - and they were together for less than three years and had their two sons before they divorced in July 2007.