RIYADH, Saudi Arabia —  Prince Bandar bin Sultan (search) of Saudi Arabia, the longest serving ambassador in Washington, is retiring after more than 20 years in his post, the Saudi foreign ministry announced Wednesday.

The new ambassador will be Prince Turk al-Faisal (search), another member of the Saudi royal family who had served as the kingdom's ambassador to Britain. Al-Faisal also served as the head of the Saudi Arabia's intelligence.

Saudi Arabia (search) has been a close Arab ally of the United States for more than 50 years and the kingdom's ambassador to Washington is a job that carries significant weight and is at the center of the oil-rich kingdom's foreign policy.

Bandar, whose father, Prince Sultan, is the Saudi defense minister, is considered the dean of Washington's diplomatic corps. He worked hard at maintaining strong ties between the United States and the conservative oil-rich monarchy.

However, since the Sept. 11, 2001, attacks the Saudis have come under pressure to counter terrorists more aggressively and block financial support going to militant groups from within Saudi Arabia.

Fifteen of the 19 hijackers were Saudis. That, and reports that some of his and his wife's charitable contributions may have ended up in the hands of two Saudis believed to have close ties to the hijackers, contributed to tensions with Washington.

"This is a war and we are in it together," Bandar said in an interview with The New York Times in November 2002. His wife, Princess Haifa al-Faisal, said she was outraged by any suggestions of a connection to terrorists. "All I wanted to do was to give some help to someone in need," she told the Times.

Bandar's resignation coincides with uncertainty about the country's ruling hierarchy. King Fahd is seriously ill, and Prince Sultan could move up in any reshuffling of authority. Bandar himself has been rumored to be in line for a top security post in Riyadh.

Bandar has been on a summer-long vacation, fueling reports that he was resigning. Saudi officials had been disputing those reports.

White House spokesman Scott McClellan called Bandar "a tireless advocate for close ties, warm relations, and mutual understanding" between the two countries.

"In troubled times U.S. presidents past and present have relied upon Ambassador Bandar's advice. In good times, they have enjoyed his wit, charm and humor," McClellan said. "The president bids Ambassador Bandar and his family a fond farewell and wishes them all the best on their return to the kingdom."

The Associated Press contributed to this report.