NEW YORK -- The man accused of being the ringleader of a terrorist conspiracy to bomb city landmarks and assassinate public officials has dismissed his law firm and apparently agreed to become a cooperating witness for the prosecution in a trial next fall, his lawyers said yesterday.

The move by the defendant, Siddig Ibrahim Siddig Ali, was reported by his lawyers, Ronald Kuby and William Kunstler.

Spokesmen for the U.S. attorney in Manhattan, Mary Jo White, could not be reached for comment on the report, and efforts to reach Howard Leader, the new lawyer for Mr. Siddig Ali, and lawyers for other defendants, were also unavailing.

The defection of Mr. Siddig Ali would be a major coup for prosecutors in the case, in which an Egyptian cleric, Sheik Omar Abdel-Rahman, and 12 men said to be his followers are charged with plotting to blow up the United Nations and other targets and to kill American and Egyptian political figures, including President Hosni Mubarak of Egypt.

Mr. Siddig Ali, a disciple of the sheik, was said to have played a central role in the plot, selecting targets, obtaining explosives and weapons, arranging for safe houses, assembling a team of conspirators and serving as a liaison with the sheik. It was Mr. Siddig Ali's voice that was most prominent on tape recordings made by a government informer who penetrated the conspiracy.

In contrast to the prosecution of four men convicted in March in the World Trade Center bombing last year -- a case that relied on fingerprints, documents and other tangible evidence -- the current case, in which no bombings or killings occurred, has centered on expected testimony by the informer, Emad Salem, and by a relatively minor defendant, Abdo Mohammed Haggag, who was accused of discussing a plan to kill Mr. Mubarak.

In a letter yesterday to federal Judge Michael Mukasey, who is scheduled to preside at the conspiracy trial starting Sept. 19, Mr. Kuby and Mr. Kunstler decried efforts by the prosecution to obtain the cooperation of Mr. Siddig Ali and other defendants in the case.

"Siddig Ali's decision represents a terrible blow, not only to him and his co-defendants, but to the entire concept of equal justice under law," Mr. Kuby told Judge Mukasey. "We have little doubt that part of the government's real agenda here is to decapitate the defense team by removing the so-called 'radical' lawyers."