Troubled pop star Britney Spears has received another blow in her custody battle with Kevin Federline - her ex has delivered a subpoena to the rehab facility she attended, according to reports. The 25-year-old singer and Federline's divorce was finalised last month (30Jul07), with the couple agreeing to joint custody of their sons Sean Preston, 23 months; and Jayden James, 11 months. However, following Spears' erratic behaviour in recent weeks, Federline is now seeking full custody of the two boys and has already subpoenaed her assistant Alli Sims, her former sidekick Shannon Funk, and her bodyguard Daimon Shippen to testify in the case. California's Promises Treatment Center - which the singer entered in February (07) for a month - was allegedly served with a subpoena on Saturday morning (18Aug07). A spokesperson from Promises refused to discuss developments, saying, "This is a confidential facility and we cannot comment." Meanwhile British newspapers report Federline has also hunted down a mystery brunette girl who was snapped frolicking with Spears in Copenhagen, Denmark, in May 2004 on her Onyx Hotel tour.