U.S. District Judge James Robertson would not comment Wednesday on his resignation, but The Washington Post reported that it stemmed from deep concern that the surveillance program Mr. Bush authorized was legally questionable and may have tainted the work of the court.
An aide to Robertson said the resignation letter submitted to Chief Justice John Roberts was not being released. Robertson did not step down from his district judgeship in Washington.
President Bush ignored questions from reporters Wednesday morning about the special court. Earlier, spokesman Scott McClellan declined to comment on Robertson's resignation.
McClellan said since the judge had not directly commented, the White House would have nothing to say, CBS News correspondent Peter Maer reports.
Robertson was one of 11 members of the secret Foreign Intelligence Surveillance Court, which oversees government applications for secret surveillance or searches of foreigners and U.S. citizens suspected of terrorism or espionage. Robertson's term was to end in May.
"This was definitely a statement of protest," said Scott Silliman, a former Air Force attorney and Duke University law professor. "It is unusual because it signifies that at least one member of the court believes that the president has exceeded his legal authority."
Ruth Wedgwood, a Johns Hopkins University professor and defender of many Bush administration policies in the terror war, said that service on the special court is voluntary.
"If Judge Robertson had strong feelings that he thought would interfere with the needed objectivity, one could understand his decision," she said.
The court was established by Congress in 1978 and its members, appointed by the chief justice, do their work in private.
Quoting colleagues of Robertson, the Post said the judge had indicated he was concerned that information gained from the warrantless surveillance under Mr. Bush's program subsequently could have been used to obtain warrants under the FISA program.
Robertson was appointed a federal judge by President Clinton in 1994. Chief Justice William Rehnquist later appointed Robertson to the FISA court as well.