WASHINGTON (Reuters) - The Bush administration cannot stop doctors from helping terminally ill patients end their lives under the nation's only physician-assisted suicide law, the    U.S. Supreme Court ruled on Tuesday.

In a stinging defeat for the administration, the high court ruled on a 6-3 vote that then-Attorney General    John Ashcroft in 2001 impermissibly interpreted federal law to bar distribution of controlled drugs to assist suicides, regardless of the Oregon law authorizing it.

The justices upheld a U.S. appeals court ruling that Ashcroft's directive was unlawful and unenforceable, and that he had overstepped his authority.

The Oregon law, called the Death with Dignity Act, was twice approved by the state's voters. The only state law in the nation allowing physician-assisted suicide, it has been used by more than 200 people since it took effect in 1997.

Under Oregon law, terminally ill patients must get a certification from two doctors stating they are of sound mind and have less than six months to live. A prescription for lethal drugs is then written by the doctor, and the patients administer the drugs themselves.