Sandra Day O'Connor, the first woman to serve on the US Supreme Court and a critical swing voter, is to retire.
The announcement by the 75-year-old sets the stage for a major political battle over her successor.
Ms O'Connor has often cast the deciding vote in the nine-member Supreme Court, leading some US commentators to call her the most powerful woman in America.
She is the first Supreme Court justice to retire since 1994, giving US President George W Bush his first chance to name a judge.
