Lawyers representing detainees at Guantanamo Bay, Cuba, say authorities at the military base have begun reading privileged attorney-client communication — in a sharp break with past practice.

Legal mail is the principal means of communication between detainees charged in military commissions and their military defense attorneys,who are based in the Washington area.

In a letter Tuesday, nine of the attorneys wrote to William K. Lietzau, deputy assistant secretary of defense for rule of law and detainee policy, to object to authorities reading their mail to clients at the detention center. They asked that the commander at Guantanamo Bay be ordered to “cease and desist the seizure, opening, translating, reading and reviewing of attorney-client privileged communications.”

A military official, who spoke on the condition of anonymity because he was not authorized to discuss the issue publicly, said that privileged mail between attorneys and their clients has always been clearly marked as such.

Previously, military personnel at Guantanamo Bay opened the mail in the presence of detainees — thus ensuring there was no contraband in the envelopes — and handed it to them without reading the contents.

Last month, the official said, Rear Adm. David B. Woods, the new commander at the prison, changed the policy and insisted on checking that the communication was relevant to commission cases.

A Pentagon spokesman declined to comment, saying the matter is the subject of ongoing litigation.

The nine military defense attorneys who signed the letter Tuesday — including the lawyer for Khalid Sheik Mohammed, the self-proclaimed mastermind of the attacks of Sept. 11, 2001 — argue that military commission rules of evidence specifically protect attorney-client-privileged material.

They have threatened to litigate the issue to “the fullest extent,” and said that doing so will stall proceedings at Guantanamo Bay.

“Violations of the attorney-client privilege are acutely egregious in the context of death penalty litigation where the Supreme Court has long held that heightened due process applies,” stated the letter, a copy of which was provided to The Washington Post. “It is important to note that the legal materials discussed are not classified.”

The attorneys, who all have security clearances, said they “are aware of the responsibilities involved in handling classified information.”

The issue first surfaced in a motion filed in the case of Abd al-Rahim al-Nashiri, a Saudi who is scheduled to be arraigned next week on charges of murder and terrorism for his alleged role in the October 2000 al-Qaeda attack on the USS Cole in Yemen.

The subject of the Oct. 26 motion, however, could be learned only from its title, which asked a military judge “to bar” the Joint Task Force at Guantanamo Bay “from violating the attorney-client privilege by reading attorney-client information.”

The motion cannot be read in full yet because it is undergoing a security review.