WASHINGTON - Henry A. Kissinger abruptly resigned as chairman of the commission that will investigate the Sept. 11 attacks yesterday, telling President Bush he could not serve if it meant revealing the clients of his consulting firm.

Bush, who aides said was surprised by the decision, vowed to quickly find a replacement.

"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," Bush said. "My administration will work quickly to select a new chairman whose mission will be to uncover every detail and learn every lesson of Sept. 11, even as we act on what we have learned so far to better protect and defend America."

Kissinger, who was appointed by Bush, is the second prominent figure on the panel to resign this week over a personal conflict. On Wednesday, former Sen. George J. Mitchell, a Democrat from Maine who had been selected by congressional Democrats to be the vice chairman, resigned from the panel because of time pressures and an unwillingness to reveal the clients of his law firm.

Congressional Democrats immediately appointed former Rep. Lee Hamilton, an Indiana Democrat, as Mitchell's replacement.

Republicans had not announced three of their appointments as tomorrow's deadline approached.

Kissinger's appointment last month revived memories of his contentious service as national security adviser and secretary of state in the Nixon administration and his role in the Vietnam War.

It also set off an argument between congressional Democrats, who argued that Kissinger must disclose his financial ties, and the White House, which said there was no need for him to do so. Kissinger did not respond to a message left at his New York office last night, but his letter to Bush said he feared that such arguments would soon engulf his firm, Kissinger Associates.

"This is a moment of disappointment for me, of course," he wrote. "My hope is that, by the decision to step aside now, the joint commission can proceed without further controversy."

Kissinger's resignation was apparently triggered by a legal opinion from Republicans and Democrats on the Senate Ethics Committee, who said Thursday that all members of the commission would have to comply with congressional financial disclosure requirements. Only hours earlier, Kissinger had met with 11 relatives of Sept. 11 victims to propose a plan under which he would reveal his clients to a third party chosen by the families who would agree not to make the names public.

The White House had fought to declare Kissinger an executive branch appointment, therefore clearing him of legislative disclosure requirements, but it dropped the legal argument after the Ethics Committee issued its opinion.

Administration officials said Kissinger called White House chief of staff Andrew H. Card Jr. yesterday afternoon to tell him of his decision after making it clear that he was prepared to release his client list.

But the officials said Kissinger had become convinced that naming his clients would not have satisfied his critics and that he would still face calls to sell his consulting company. In the end, the officials said, Kissinger decided that severing his ties to his company was too big a price to pay for returning to public service.

A friend of Kissinger's said last night that the former secretary of state accepted the job in the spirit of public service and out of unwillingness to say no to the president, never anticipating that his client list would become a point of contention. But after Democrats, newspaper editorials and family members demanded the disclosure, Kissinger faced the choice of serving the president or breaking the confidentiality agreements he signed with his clients.

"If you have confidentiality agreements, you can't just throw them away," said the friend, who spoke on the condition of anonymity.

But Sen. Harry Reid of Nevada, the Democratic whip, said it was inevitable that someone with Kissinger's global ties would have to step down from a commission looking into the conduct of foreign countries and domestic intelligence agencies.

"There were too many conflicts of interest for him to lead this task," said Reid, who is chairman of the Ethics Committee. "I knew he would never disclose that information. Now the country and the families deserve someone who won't be afraid to lead this commission."

Reid criticized the White House for trying so hard to protect Kissinger's privacy.

"The pressure put on the Ethics Committee by the White House was really untoward," he said. "They were calling and berating our staff, saying he didn't have to file because he worked for the executive branch. I mean, come on. What were they trying to hide? Finally, he realized he couldn't hide it anymore."

Kissinger's friend said there were "no deep dark secrets," with no clients in the Middle East or in government.

Leaders of the families who fought to create the commission said they were not particularly disappointed by the resignation.

"I was always a little uneasy with him, mostly because I never thought he would be a very dogged investigator," said Stephen Push, a leader of Families of Sept. 11. "Of course, I wanted to know who his clients were, but I also thought he just wasn't the right personality for the job."

It is unlikely that the White House will find a replacement for Kissinger by tomorrow - the date, under the law establishing the commission, by which all 10 members must be in place. There is no penalty for missing the deadline; it was set to avoid a lengthy appointment process.