Advisor to Ronald Reagan;

Born: October 23, 1931; Died: August 6, 2013.

William P Clark, who has died of Parkinson's disease aged 81, rose from campaign volunteer to become one of President Ronald Reagan's most trusted advisers.

Once called the second most powerful man in the White House, he regularly bonded with the President while riding horses at Camp David and is said to have been responsible for pushing him towards taking more hardline positions on issues such as arms control and intervention in Central America.

Clark began working for Reagan by managing his 1966 campaign to become governor of California and worked for him in Sacramento, rising to the position of executive secretary, before accepting a judgeship with the San Luis Obispo County Superior Court.

Reagan later appointed him to the state appellate court in Los Angeles, and then the state Supreme Court, before he moved to Washington to serve as deputy secretary of State and national security adviser.

Mr Clark was national security adviser when Reagan manoeuvred the Soviet Union toward arms control, and he was a key player in Reagan's philosophy of peace through strength.

The New York Times said Mr Clark had more access to Reagan than anyone else. "They had very similar ideas about what ought to be done," said Edwin Meese, who served as counsellor to President Reagan and then as attorney general. "And they also knew and understood one another very well, having worked together back in the California days."

Mr Clark then served as interior secretary for nearly two years, replacing unpopular department head James Watt before returning to his private law practice and business consulting firm.

Born in 1931 in Oxnard to a family of lawmen (his grandfather Robert was Ventura County sheriff and a US marshal; his father William Sr. was the police chief of Oxnard), Clark served in the Army Counter-Intelligence Corps in Europe in the mid-1950s. He attended both Stanford University and Loyola Law School without earning degrees, but nevertheless passed the bar exam.

"That's been pointed out throughout his career that he finished neither college nor law school, but be that as it may he did just fine," his son Paul said.

After retiring from public life, Mr Clark and his wife Joan designed and built a chapel in Shandon, which they donated to the community. A devout Catholic, he also became a strong opponent of abortion.

He is survived by five children. His wife passed away four years ago.