The pair are said to have split a week ago because of their busy work schedules.
Britney, 20, has been going out with the N'Sync star for years, and they were engaged to be married.
They were previously thought to be one of the steadiest couples in showbiz.
Friends for 11 years
In January, Britney said: "We do want to get married and have kids and stuff like that - however, maybe in 15 years time."
Britney and Justin have been mates since they met on the Disney TV show, The Mickey Mouse Club, back in 1993.