For 15 years, Brian Jones headed the unit that analyzed nuclear, biological and chemical weapons intelligence data for Britain's Defense Ministry, a job so secret that even his mother did not know what he did.

Then one day last August, he and his wife, Linda, turned on the 6 o'clock news and saw that the lead item was about a confidential letter he had written to his supervisor.

"As it appeared, our chins fell closer to the floor," he recalled with rueful smile. "We had visions of a scrum of journalists gathered around the house. We really didn't want to be in the public arena in this way."

The letter appeared to contradict claims by Prime Minister Tony Blair and other senior officials that there was no significant dissent within the intelligence community over a controversial dossier that the government published in September 2002 to detail its charge that Iraq had weapons of mass destruction. Without telling Jones, the ministry had turned over the letter to a public inquiry, placing him at the heart of a highly charged dispute over intelligence, politics and the invasion of Iraq.

Since then, the formerly anonymous insider has become a public dissident in Britain, arguing that government officials exaggerated the threat posed by Iraq, overstated the evidence and backed up their conclusions with last-minute raw intelligence that they concealed from Jones and his analysts. The debate has echoed a similar one that broke out in the United States after occupation troops failed to find weapons of mass destruction, whose purported existence was the war's prime justification.

In Britain, two public inquiries have rejected the claim that the government deliberately distorted the spy agencies' reports. Still, Jones's views were exonerated last month when one of the inquiries, the Butler Commission, concluded he had been right to raise concerns about the dossier and that he and his analysts should have been shown the last-minute intelligence, which was later withdrawn as unreliable.

He contends that the government's mistaken claim that Iraq possessed weapons of mass destruction has done serious harm to the credibility of the intelligence community and to international efforts against such weapons.

"The damage has been considerable, and I think it will continue until exactly what went wrong is clarified," said Jones, sitting in his living room in southwestern England, with Linda at his side, in his first interview with a non-British news organization. Jones, who just turned 60, took early retirement a few months after the dossier was published.

He depicts himself as a reluctant whistle-blower whose original motivation was to protect himself and his colleagues bureaucratically rather than to make a stand on principle. Jones expressed his objections in writing, he says, so that no investigator could come along later and accuse him and his staff of signing off on a flawed dossier. His tale also sheds light on Britain's ultra-secret intelligence establishment's methods, calculations and vulnerabilities.

"In some ways, the whole controversy has done more damage here than in the U.S.," said Gary Samore, a weapons expert at the London-based International Institute for Strategic Studies. "There have been so many intelligence failures in the United States that the reputation of CIA was already dented, whereas here there is still a lot of mystique about MI6," Britain's secret intelligence service.

"It will be a long time before the intelligence community produces this kind of dossier again," Samore added.

When Jones, who has a doctorate in metallurgy, first took over as head of the technical intelligence branch of the agency in 1987, he recalled, his unit largely consisted of experts in weapons materials and chemistry -- with just two men whose specialty was chemical and biological warfare sitting in a corner.

That all started to change in the late 1980s, after the Soviet Union disclosed that it had produced chemical and biological weapons and Iraq began using chemical weapons against Iran. Over the next few years, Jones's unit began to concentrate on such weapons, and in the mid-1990s, it added nuclear components as well. It became known as the primary shop within the British intelligence establishment where data on weapons of potential mass destruction were analyzed and assessed.

MI6 "are the collectors," not the analysts, Jones said. "One of the great sins in intelligence is to allow the people who collect the intelligence to assess its quality, because the very fact they collected it means they are biased. That's why you have analysts who are independent."

During the summer of 2002, Blair had spoken about producing an Iraqi weapons dossier, but when Jones went off to Greece for vacation in late August, the project seemed to be on hold. He returned to the office on Sept. 18 to find it in full swing, with publication less than a week away -- and parts of his staff in dismay. Jones's most senior chemical weapons specialist was particularly concerned that the dossier's drafters, who worked for the cabinet-level Joint Intelligence Committee, had ignored his concerns about the document's claims concerning chemical weapons.

"He felt the language was much too strong," Jones recalled. "He was saying he could find no conclusive evidence that Iraq had produced [a] chemical warfare agent or weapons."

The dossier was being crafted by the JIC with help from Blair's political and press advisers. In its forward, Blair stated: "I believe the assessed intelligence has established beyond doubt that Saddam has continued to produce chemical and biological weapons . . . I am in no doubt the threat is current and serious."

The specialists were also skeptical about a claim, based upon a single source reporting information that he said he had heard from an Iraqi military officer, that Iraq could launch chemical or biological weapons within 45 minutes of an order.

Jones talked to his experts individually on the day he returned, then held a 30-minute session with all of them the following day, Sept. 19.

Among those who sat in was David Kelly, a government weapons expert who was not a member of Jones's staff but was a frequent visitor. After the session, Jones took the unusual step of writing a note to his immediate superior, Tony Cragg, registering his group's objections.

Filing a written dissent was a highly unusual step. "The whole culture of the JIC is that it produces consensus," Jones said. "At the end of the day, we want something that we're all agreed with. So if you are actually putting down a dissent, you're saying all this effort has failed. I maybe did it once, maybe twice before in 15 years. You don't do it lightly."

Cragg later told the Hutton inquiry, the other public investigation touching on Iraq-related intelligence, that he was surprised to read Jones's memo. He took up the matter with his superior, the chief of defense intelligence, and the two men decided the matter had already been resolved satisfactorily. The dossier went forward as written. "I was content for it to go to print," Cragg testified.

Word was passed to Jones that the reason his concerns were being ignored was that a new piece of intelligence had arrived a few days earlier that backed up the 45-minute claim. The source was said to be so sensitive that Jones and his analysts were not allowed to see the material.

"I was very wary," Jones recalled. "My job was all about being suspicious -- suspicious of other people, suspicious of the information you receive. What sort of information could this be that pops up all of a sudden that answers the dozen questions you've got? I thought our dissent was being finessed away."

In late May 2003, with the invasion completed and no weapons of mass destruction found, BBC defense correspondent Andrew Gilligan reported that an unnamed source had told him of great unrest within the intelligence community about the dossier, especially its 45-minute claim. Gilligan mistakenly reported that Blair's aides probably knew the claim was wrong, but otherwise his report accurately captured the feelings of Jones and his staff.

The broadcast set off a bitter controversy between the BBC and the government, which denied the story, as well as a hunt for Gilligan's source that led eventually to Kelly. Kelly was forced to testify before Parliament's foreign affairs committee and subsequently committed suicide, in July 2003. Jones says it was only then that he realized that Kelly must have been Gilligan's informant, and that his longtime colleague had been talking about the discontent he had heard expressed by Jones's staff the previous September.

Jones soon found himself under the same spotlight that had burned Kelly. In early July, after Foreign Secretary Jack Straw told the foreign affairs committee that he was unaware of any significant discontent within the intelligence community, Jones wrote a letter to Cragg's successor, reminding him of the previous note of dissent and asking whether he should inform the committee of the error. Less than two months later, the Hutton inquiry into Kelly's death made the note public, although with Jones's name expunged.

The Joneses expected Brian's name to leak -- and reporters to descend on their home. "We made arrangements to move out quickly if we had to -- to stay with one of Brian's sisters," Linda Jones recalled. "The bed there has been made up ever since."

But when the time came to testify, the Joneses decided that Brian would be better off coming forward publicly to preempt the kind of media frenzy that had engulfed Kelly. "We came to the conclusion that if Brian appeared as himself, the press would have his name and his photograph and that would remove the need for a scoop," she said.

Appearing before the House of Commons last month, a chastened Blair conceded that there should have been clear procedures for senior intelligence officers such as Jones to take their objections directly to the Joint Intelligence Committee. Blair also conceded that he and his government had made mistakes in the period preceding the war.

"What I do not accept," he quickly added, "is that it was a mistake to go to war."

Two public inquiries and two parliamentary reports have dismissed claims that the government deliberately exaggerated the threat, and opinion polls indicate that the prime minister is on track to win a third term. John Scarlett, the senior official responsible for the dossier, recently became head of MI6.

Kelly is dead, the two BBC senior executives who initially stood by Gilligan's report were forced from their posts, and John Morrison -- a former deputy chief of defense intelligence who publicly criticized the government -- recently lost his job with the committee that has oversight of British intelligence.

Jones said Scarlett's promotion might make it harder for the intelligence services to regain their public standing. "I can't comment on his competence, but his association with a major intelligence failure is unavoidable," said Jones. "Intelligence is about credibility, and we're in a situation with al Qaeda and the terrorist threat where credibility is absolutely vital. It makes things that much more difficult for credibility to be reestablished, with John Scarlett in that position."

What really troubles him, Jones said, is that British intelligence and its sister agencies in other countries failed to accurately assess Iraq's weapons programs despite a clear mandate to do so. This suggests to him the limitations of intelligence and compliance monitoring, including inspections.

"After Iraq lost the first Gulf war, we had greater access through inspectors than any of the existing arms control treaties would give us, and still we failed collectively through intelligence and through compliance monitoring to get the right answer," he said.

"The truth is, we need to do a whole lot better than we did in Iraq," Jones said, "but I am not confident that what is required can actually be achieved."

Brian Jones depicts himself as a reluctant whistle-blower whose original motivation was to protect himself and his colleagues bureaucratically. Prime Minister Tony Blair spoke in the House of Commons on July 14 after a commission said Jones had been correct to raise concerns about a dossier.