In the report to the Security Council Monday, Hans Blix said U.N. inspectors "did not find evidence of the continuation or resumption of programs of weapons of mass destruction or significant quantities of proscribed items."
But, he said, the inspectors had many questions about its chemical and biological programs when they left shortly before the March 20 invasion.
The report comes amid growing controversy over the pre-war intelligence that Saddam Hussein maintained massive stockpiles of illegal weapons and the systems to produce and deploy them.
So far, only two suspected mobile biological factories have been found.
The British government is under pressure to prove it did not fake weapons evidence to justify war. The Bush administration's claims are also under scrutiny.
In other developments:
Prime Minister Tony Blair faced more pressure Tuesday to explain his government's claims that Saddam had weapons of mass destruction, and some lawmakers said a formal inquiry seemed increasingly likely.
Controversy has focused on a government dossier, published in September, outlining evidence that Iraq had weapons of mass destruction and plans to deploy them on 45 minutes' notice.
More than 50 Labor lawmakers have signed a Commons motion drafted by Peter Kilfolye, a former Labor defense minister, calling on the government to publish the evidence behind the dossier.
Tony Wright, a member of Blair's Labor Party and chairman of the Commons Public Administration Committee, said an inquiry was now "almost inevitable."
According to the Guardian newspaper, Blair said Monday that "the idea that we doctored intelligence reports in order to invent some notion about a 45-minute capability for delivering weapons of mass destruction is completely and totally false."
The United States and Britain used the claim that Iraq had illegal weapons programs as a major reason for the war that toppled Saddam Hussein's regime.
In Rome Monday, Secretary of State Colin Powell said: "There were weapons of mass destruction in Iraq. It wasn't a figment of anyone's imagination."
He cited Baghdad's use of the weapons in the war against Iran and against Iraqi Kurds in the 1980s, and the discovery of such weapons by U.N. inspectors after the 1991 Gulf War.
President Bush said this weekend that weapons had been found. As evidence, however, he pointed to two suspected mobile biological laboratories, which both the Pentagon and American weapons hunters have said do not constitute arms.
Blix said in his 40-page report that Iraq denied any such units existed and had provided U.N. inspectors "with pictures of legitimate vehicles, which, they suggested, could have led to the information."
He noted, however, that "none of the vehicles in these pictures look like the trucks recently described and depicted" by the U.S.-led teams hunting for weapons.
U.N. inspections uncovered "a small number of undeclared empty chemical warheads which appear to have been produced prior to 1990," he said.
These were destroyed along with a few other proscribed items and some 70 Al Samoud 2 missiles with a range beyond the 92-mile limit allowed under U.N. resolutions.
While Iraq's cooperation with U.N. inspectors started improving in late January and inspectors got "a better understanding of past weapons programs," Blix said, "little progress was made in the solution" of outstanding disarmament issues.
Extensive excavations by the Iraqis, which were witnessed by U.N. inspectors, showed that Iraq had destroyed a large number of R400 bombs containing a biological agent, as it had claimed. But the excavations couldn't verify the amount of agent produced or destroyed, he said.
Blix said U.N. inspectors also didn't have time to complete their investigation on whether pilotless Iraqi drones or interview Iraqis involved in weapons destruction.