Former secretary of state Henry A. Kissinger abruptly resigned yesterday as head of a new commission investigating the Sept. 11, 2001, terrorist attacks, complaining in a bitter letter to President Bush that concerns about conflicts of interest could "significantly" delay the panel's work.

The departure was the second in three days from the panel, and ended two weeks of intense political infighting over whether Kissinger's controversial past and influential business contacts would affect the commission's findings.

Kissinger had resisted calls from Senate Democrats that he publicly disclose his business clients to guard against conflicts of interest.

The decision to quit was "a moment of disappointment for me," Kissinger wrote to Bush. "For over half a century, I have never refused to respond to the call from a president," he said. "Nor have I ever put my personal interests ahead of the country's interests."

He said that "in the end," he would have abided by whatever financial disclosure rules were applied to other members of the commission. But he feared that "the controversy would quickly move to the consulting firm I have built and own."

"To liquidate Kissinger Associates cannot be accomplished without significantly delaying" the work of the commission, he wrote, referring to his New York-based company. "I have, therefore, concluded that I cannot accept the responsibility you proposed."

Two days earlier, former senator George J. Mitchell (D-Maine) announced his withdrawal as vice chairman of the commission, citing in part suggestions that he should sever ties to his law firm.

The 10-member, bipartisan commission is to follow up on a joint House-Senate probe examining intelligence failures before Sept. 11, 2001, but it has yet to achieve a full slate of appointments. It has been the focus of intense political infighting since being created by Congress last month.

The withdrawal came as a surprising and disappointing setback for the White House, where officials had been convinced that Kissinger's name would bring credibility to an enterprise they had once resisted.

But the appointment had also prompted a steady stream of objections, including some from relatives of victims in the terrorist attacks. They questioned Kissinger's reliability and urged him to disclose his client list.

Kissinger met with 11 members of victims' family groups Thursday, telling them that he did not believe he had any conflicts of interest and promising to provide the families personally with details. But, according to several of the meeting's participants, he indicated he did not intend to release the information publicly.

Stephen Push, a leader of Families of Sept. 11th, said yesterday he found Kissinger's resignation "very puzzling."

"He told us that he had no conflicts, yet he's apparently resigning from an opportunity to serve his country so his clients can remain anonymous," Push said. "It's disturbing that we've had two high-profile resignations in a row."

But Kristen Breitweiser, another relative active in the follow-up investigations to Sept. 11, 2001, said it was "admirable for him to acknowledge that he couldn't get around the conflict issue."

"We all want the report . . . to be the final say on the issue," she said. "There shouldn't be a cloud over it."

Kissinger called White House Chief of Staff Andrew H. Card Jr. yesterday to tell him of the decision, sources said. Bush said in a statement that he accepted Kissinger's resignation "with regret," and that "his chairmanship would have provided the insights and analysis the government needs to understand the methods of our enemies and the nature of the threats we face."

A senior aide said the administration hopes to find a replacement by Christmas, with a search to be conducted by Card, national security adviser Condoleezza Rice and presidential personnel director Clay Johnson.

Democrats have named their five appointments to the panel, including former Indiana congressman Lee H. Hamilton as vice chairman to replace Mitchell. Congressional Republicans have named former senator Slade Gorton (Wash.) and have until Sunday to appoint three more. Families of victims have lobbied Republicans to appoint former senator Warren B. Rudman, but Republican leaders have indicated they oppose naming him because of his maverick reputation. Instead, House Speaker J. Dennis Hastert's office said former Illinois governor Jim Thompson and former trade representative Carla Hills may be named.

A senior White House official said the departures of Mitchell and Kissinger show that onerous disclosure demands by Congress provide a "disincentive for good people to serve in government."

"As he makes clear in his letter, he was prepared to release all his client lists," the official said, referring to Kissinger. "But in the end, he decided that it wouldn't be good enough; that people would want him to sever ties with his clients; that to serve your country you can't make a living."

Kissinger's appointment, announced Nov. 27, was immediately criticized by some Democrats and liberal commentators, who said that Kissinger's polarizing role in directing foreign policy during the Nixon and Ford years made him unfit to lead a panel aimed at unearthing unpleasant truths.

In the weeks that followed, debate on Capitol Hill shifted to whether Kissinger and other commission members should be required to disclose their recent business contacts to avoid potential conflicts of interest. Some suggested that members should sever ties to their current businesses, as well.

The fight came to a head this week, when the White House informed the Senate Select Committee on Ethics that Bush administration lawyers concluded that Kissinger, as an unpaid presidential appointee, did not have to abide by congressional disclosure rules.

The Congressional Research Service, by contrast, issued a report last week concluding that all the panel's members, including Kissinger, would be required to identify clients who paid them more than $5,000 over the past two years.

Hamilton, in a statement yesterday, said that "all five Democratic members support complete disclosure, and we will each comply fully" with congressional requirements for conflicts-of-interest disclosures.

Staff writer Mike Allen contributed to this report.

Henry A. Kissinger's departure from the bipartisan commission was the second in three days. President Bush, who tapped Henry A. Kissinger to lead the independent commission despite some initial objections, said in a statement that he accepted the resignation of the former secretary of state "with regret."