The 'Till The World Ends' singer - who is currently a judge on 'The X Factor' USA - is working out like crazy to shape up for their big day which is said to be taking place within weeks of the current series of the reality TV show finishing.
A source told The Sunday Mirror: ''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'.''
The singer got engaged to Jason, her former agent, last December and the couple recently purchased a new £5.3 million house together.
The 30-year-old - 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.
News of the popstar's upcoming wedding follows reports that her father Jamie was ''scared to death'' at the time of her psychiatric episode in 2008 and thought she would never recover.
Speaking in court during the second day of her trial against former manager Sam Lutfi, attorney Leon Gladstone said: ''Jamie was living in constant fear that he was going to look at the TV one day and see Britney was dead.''
The court battle kicked off on Thursday (18.10.12) and saw Sam - who is suing Britney and her parents for breach of contract and libel - make a series of damning allegations about drugs, while her legal conservator Jamie claimed Sam's toxic influence was to blame for her breakdown, during which she lost custody of her two children and shaved her head.
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