WASHINGTON — Richard Perle, one of the most outspoken advocates for invading Iraq, has quietly resigned from the Defense Policy Board, an influential bipartisan Pentagon advisory group.

Perle informed Secretary of Defense Donald H. Rumsfeld that he was quitting the board in a letter dated Feb. 18, although a week later a Pentagon list of board members still included him. A copy of the letter was obtained by Knight Ridder.

Perle's resignation comes as President Bush, who had hoped to ride popular support for the wars in Afghanistan and Iraq to a second term, finds his administration facing a growing number of congressional, legal and internal investigations into dubious prewar intelligence on Iraq and lucrative contracts for Iraqi reconstruction.

In his letter, Perle said he was resigning after 17 years on the board so that the Bush administration and the Department of Defense would no longer be associated with his outspoken views on Iraq and other matters.

"We are now approaching a long presidential election campaign, in the course of which issues on which I have strong views will be widely discussed and debated," Perle wrote. "I would not wish those views to be attributed to you or the president at any time, and especially not during a presidential campaign."

Perle didn't return a telephone call seeking comment on his resignation, and a Pentagon spokesman would confirm only that he had resigned.

In recent weeks, Perle has called for the resignation of CIA Director George Tenet, criticized Secretary of State Colin Powell and other current and former senior U.S. officials as "soft-liners" and urged the Bush administration to consider pulling out of the United Nations if the agency doesn't legalize pre-emptive attacks on states that harbor terrorists.

Perle also is a prominent supporter and close friend of Ahmad Chalabi, a member of the U.S.-appointed Iraqi Governing Council who's the subject of a Senate Intelligence Committee investigation into exaggerated and fabricated intelligence about Iraqi weapons programs and ties to Osama bin Laden.

In the run-up to the invasion, Perle advocated installing Chalabi as Iraq's interim leader, and he told Knight Ridder in a July 2003 interview that CIA and State Department opposition to this plan led to the insurgency against the U.S.-led occupation.

Perle served as the board's unpaid chairman from July 2001 until March 2003, when he quit the post while the Pentagon inspector general looked into whether he had violated ethics rules by representing companies that had dealings with the Defense Department. He was cleared of any breaches and remained a board member.

A British newspaper reported last week that questions also have been raised about $3 million in bonuses that Perle reportedly received in 2000 and 2001 from the firm Hollinger International. The company is investigating undisclosed payments to its executives, according to The Times of London.

Perle's resignation coincides with the publication of a book he co-authored calling for "bold action" against Iran, North Korea and other "sponsors of terrorism," including U.S. ally Saudi Arabia. The book, "An End To Evil: How to Win The War on Terrorism," co-authored with David Frum, argues that Iran and North Korea "present intolerable threats to American security."

"We must move boldly against them both and against all the other sponsors of terrorism as well: Syria, Libya and Saudi Arabia. And we don't have much time," the book argues.

Although the Defense Policy Board is a bipartisan advisory committee that has no role in making policy, Perle was an influential member of the group, which includes Vice President Dick Cheney, Deputy Secretary of Defense Paul Wolfowitz, Undersecretary of Defense Douglas Feith and I. Lewis Libby, Cheney's chief of staff, who were the most outspoken proponents of a pre-emptive attack on Iraq.

In his resignation letter, Perle wrote that too often his views have been seen as those of the administration's. "Many of the ideas in that book are controversial and I wish to be free to argue for them without those views or my arguments getting caught up in the campaign," he wrote.

Perle and Frum are fellows at the American Enterprise Institute, a Washington think tank that's a leading champion of conservative causes. Cheney was a fellow there; his wife, Lynne, still is.

Perle served as an assistant secretary of defense during the Reagan administration. He earned the nickname "Prince of Darkness" for his passionate opposition to detente with the former Soviet Union and to arms-control treaties, which he considered unenforceable.