Kevin Federline's defense team have subpoenaed the rehab clinic where his ex-wife Britney Spears was a patient to testify in their custody battle.

Kevin's attorney Mark Vincent Kaplan confirmed "an administrative executive" at Malibu's Promises center - where the 25-year-old troubled singer checked into earlier this year after shaving off her hair - was served legal papers on Saturday.

There was a time when it looked like Kevin Federline wouldn't have a shot in Hollywood of getting full custody of his children.

But then Britney began an ever-continuing meltdown and K-Fed found work at a radio station, set up a family home and stopped trying to milk his 15 minutes of fame with bachelor trips to Vegas.