Britney Spears and her former husband Kevin Federline have become best friends, according to her sister.

Jamie Lynn Spears told Hello! Kevin, who split from Britney three years ago, had been taking care of the couple's two children on the singer's US tour.

Kevin, who has custody of Sean Preston, 3, and Jayden James, 1, is also apparently on good terms with Britney's parents.

"Kevin has been great, as always,” Jamie Lynn said.

“He and Britney have become best friends. She knows how much he cares about the boys and he knows the same is true of her."

Britney and Kevin married in 2004 after just six months together, but announced plans to divorce in November 2006.

Last year, Kevin was awarded full custody of the couple's two children as the singer battled to control her personal life.

Britney is due to head to Europe in June to begin the international leg of her Circus, world tour.

Britney Spears and Kevin Federline