British pop star Robbie Williams has entered a rehabilitation centre to be treated for a dependency on prescription drugs - on his 33rd birthday. The FEEL singer has been admitted to a rehab clinic in the US, according to his spokeswoman BRYONY WATTS. In September (06), an exhausted and stressed Williams cancelled the Asian leg of his world tour. Williams has previously been in rehab in the mid-1990s after he left boy band Take That, when he was treated for addictions to cocaine, ecstasy and alcohol. He has also been treated for depression in the past.