Britney Spears has realised her marriage to Kevin Federline was a mistake - because she tied the knot for all the "wrong reasons".

The pop superstar stunned fans when she announced her engagement to backing dancer Federline in 2004 after just three months together.

They wed two months later in a surprise ceremony in California, officially becoming man and wife in October.

Spears filed for a divorce from Federline in 2006, citing irreconcilable differences as the reason behind the split. The divorce was finalised in July last year.

And the singer, who gushed about her love for Federline in their 2005 MTV reality show Britney + Kevin: Chaotic, has confessed she didn't know what she was letting herself in for when she walked down the aisle with her former beau.

In a candid new documentary, Britney: For The Record, the singer says, "I think I married for the wrong reasons.

"Instead of following my heart and doing what made me really happy, I just did it for the sake of the idea of everything."

Federline was granted sole custody of the former couple's sons Sean Preston, three, and two-year-old Jayden James after Spears had a series of public meltdowns, including a headline-grabbing incident when she shaved off her hair.