The action by U.S. District Judge James Robertson stemmed from deep concern that the surveillance program that Bush authorized was legally questionable and may have tainted the work of the court that Robertson resigned from, say two associates of the judge.

Robertson was one of 11 members of the secret Foreign Intelligence Surveillance Court (FISA), which oversees government applications for secret surveillance or searches of foreigners and U.S. citizens suspected of terrorism or espionage.

Colleagues of Robertson say the judge is concerned that information gained from the warrantless surveillance under Bush's program could have then been used to obtain warrants under the FISA program.

Robertson, without providing an explanation, stepped down from the FISA court in a letter late Monday to Chief Justice John Roberts.

Supreme Court spokeswoman Kathy Arberg said early Wednesday she had no information to offer on the matter.

Robertson was appointed a federal judge by President Clinton in 1994. Chief Justice William Rehnquist later appointed Robertson to the FISA court as well.

Robertson has been critical of the Bush administration's treatment of detainees at the U.S. naval prison at Guantanamo Bay, Cuba, most memorably in a decision that sidetracked the president's system of military tribunals to put some detainees on trial.