The officer, an employee at the agency for more than 20 years, including several years in a clandestine unit assigned to gather intelligence related to illicit weapons, was fired in 2004.

In his lawsuit, he says his dismissal was punishment for his reports questioning the agency's assumptions on a series of weapons-related matters. Among other things, he charged that he had been the target of retaliation for his refusal to go along with the agency's intelligence conclusions.

Michelle Neff, a C.I.A. spokeswoman, said the agency would not comment on the lawsuit.

It was not possible to verify independently the former officer's allegations concerning his reporting on illicit weapons.

His information on the Iraqi nuclear program, described as coming from a significant source, would have arrived at a time when the C.I.A. was starting to reconsider whether Iraq had revived its efforts to develop nuclear weapons. The agency's conclusion that this was happening, eventually made public by the Bush administration in 2002 as part of its rationale for war, has since been found to be incorrect.

While the existence of the lawsuit has previously been reported, details of the case have not been made public because the documents in his suit have been heavily censored by the government and the substance of the claims are classified. The officer's name remains secret, in part because disclosing it might jeopardize the agency's sources or operations.

Several people with detailed knowledge of the case provided information to The New York Times about his allegations, but insisted on anonymity because the matter is classified.

The former officer's lawyer, Roy W. Krieger, said he could not discuss his client's claims. He likened his client's situation to that of Valerie Wilson, also known as Valerie Plame, the clandestine C.I.A. officer whose role was leaked to the press after her husband publicly challenged some administration conclusions about Iraq's nuclear ambitions. (The former officer and Ms. Wilson worked in the same unit of the agency.)

"In both cases, officials brought unwelcome information on W.M.D. in the period prior to the Iraq invasion, and retribution followed," said Mr. Krieger, referring to weapons of mass destruction.

In court documents, the former officer says that he learned in 2003 that he was the subject of a counterintelligence investigation and accused of having sex with a female contact, a charge he denies. Eight months after learning of the investigation, he said in the court documents, the agency's inspector general's office informed him that he was under investigation for diverting to his own use money earmarked for payments to informants. He denies that, too.

The former officer's claims concerning his reporting on the Iraqi nuclear weapons program were not addressed in a report issued in March by the presidential commission that examined intelligence regarding such weapons in Iraq. He did not testify before the commission, Mr. Krieger said.

A former senior staff member of the commission said the panel was not aware of the officer's allegations. The claims were also not included in the 2004 report by the Senate Select Committee on Intelligence on prewar intelligence. He and his lawyer met with staff members of that Senate committee in a closed-door session last December, months after the report was issued.

In his lawsuit, the former officer said that in the spring of 2001, he met with a valuable informant who had examined and purchased parts of Iraqi centrifuges. Centrifuges are used to turn uranium into fuel for nuclear weapons. The informant reported that the Iraqi government had long since canceled its uranium enrichment program and that the C.I.A. could buy centrifuge components if it wanted to.

The officer filed his reports with the Counter Proliferation Division in the agency's clandestine espionage arm. The reports were never disseminated to other American intelligence agencies or to policy makers, as is typically done, he charged.

According to his suit, he was told that the agency already had detailed information about continuing Iraqi nuclear weapons efforts, and that his informant should focus on other countries.

He said his reports about Iraq came just as the agency was fundamentally shifting its view of Iraq's nuclear ambitions.

Throughout much of the 1990's, the C.I.A. and other United States intelligence agencies believed that Iraq had largely abandoned its nuclear weapons program. In December 2000, the intelligence agencies issued a classified assessment stating that Iraq did not appear to have taken significant steps toward the reconstitution of the program, according to the presidential commission report concerning illicit weapons.

But that assessment changed in early 2001 - a critical period in the intelligence community's handling of the Iraqi nuclear issue, the commission concluded. In March 2001, intelligence indicating that Iraq was seeking high-strength aluminum tubes from China greatly influenced the agency's thinking. Analysts soon came to believe that the only possible explanation for Iraq's purchase of the tubes was to develop high-tech centrifuges for a new uranium enrichment program.

By the following year, the agency's view had hardened, despite differing interpretations of the tubes' purposes by other intelligence experts. In October 2002, the National Intelligence Estimate, produced by the intelligence community under pressure from Congress, stated that most of the nation's intelligence agencies believed that Iraq was reconstituting its nuclear program, based in large part on the aluminum tubes.

The commission concluded that intelligence failures on the Iraqi nuclear issue were as serious and damaging as any other during the prelude to the Iraqi war. The nation's intelligence community was wrong "on what many would view as the single most important judgment it made" before the Iraq invasion in March 2003, the commission report said.

Mr. Krieger said he had asked the court handling the case to declassify his client's suit, but the C.I.A. had moved to classify most of his motion seeking declassification. He added that he recently sent a letter to the director of the F.B.I. requesting an investigation of his client's complaints, but that the C.I.A. had classified that letter, as well.

Most of the details of the case, he said, "were classified by the C.I.A., not to protect national security but to conceal politically embarrassing facts from public scrutiny."