Clarke, in an E-mail sent overnight Thursday to colleagues, cited damage from the weekend's infection that struck hundreds of thousands of computers worldwide, slowing E-mail and Web surfing and even shutting down some banking systems. He called the attacking software "a dumb worm that was easily and cheaply made.
"More sophisticated attacks against known vulnerabilities in cyberspace could be devastating," Clarke wrote. "As long as we have vulnerabilities in cyberspace and as long as America has enemies, we are at risk of the two coming together to severely damage our great country."
A spokeswoman confirmed Clarke's E-mail as authentic. It was forwarded by the FBI's National Infrastructure Protection Center to operators of Internet early-warning centers.
The Associated Press, citing people familiar with Clarke's plans, reported his decision to resign on Jan. 24. Clarke has spent 11 years in the White House across three administrations, and he was the president's counterterrorism coordinator at the time of the Sept. 11 terror attacks.
Clarke has focused most recently on preventing disruptions to important computer networks from Internet attacks, compiling recommendations to improve security into a "National Strategy to Secure Cyberspace." In his e-mail, Clarke urged companies and government agencies to adopt these recommendations.
He said it was "essential to the health of the nation's economy and the security of the country."
Clarke indicated he would seek a job in the private sector, after spending three decades inside the government. He worked at the Departments of Defense and State, then was hired at the White House.
"I hope now to learn how to contribute to these issues as a private citizen," Clarke wrote.