Richard Clarke, the White House official who has been leading efforts to fortify security on the Internet, plans to resign within the next few weeks from his position as a special adviser to the president, according to sources who have been briefed about his plans.

Clarke had been considered for a mid-level job in the new Department of Homeland Security but he turned down advances. Clarke has been outspoken in his concern about the security of computers connected to the Internet, and he has expressed frustration that he did not get more support for his cybersecurity efforts.

But sources close to Clarke insist that he is not leaving because of that. They said he simply wants to pursue new challenges outside government after 30 years of public service and a stint as deputy assistant secretary in the State Department.

Clarke is perhaps most famous for being the terrorism czar under both Presidents Bill Clinton and Bush and was in this post when the planes crashed into the Pentagon and World Trade Center towers on Sept. 11, 2001.

Years before Osama bin Laden and al Qaeda became household names, Clarke had been warning about what he believed to be inevitable attacks by terrorist groups. Some in government thought his dire predictions were exaggerated. But he turned out to be right.

After the attacks, Bush named Clarke head of a new White House Office of Cyberspace Security, reporting to both the Office of Homeland Security and the national security adviser.

He's been traveling the country urging corporate executives to spend more money on securing their networks, urging legislation for more funding for cybersecurity, and holding town halls to get citizens involved.

His most ambitious endeavor has been in trying to get government and industry to agree on a set of recommended best practices for securing networks. The report outlining the strategy is scheduled to be released as early as next week. Clarke is planning to submit his letter of resignation soon afterward.

Reaction to Clarke's crusade to improve security on the Internet has been mixed.

When a draft of the national strategy was released, there was criticism that it went too easy on corporate America. But others praised him for having the guts to press executives such as Bill Gates and John Chambers to do more to secure their products. Indeed, both Microsoft Corp. and Cisco Systems Inc. began company-wide security initiatives shortly after executives met with Clarke.

"I don't think he did as much as he wanted to given his resources and authority. But I think we'd be worse off without the efforts he put into place," said Eugene H. Spafford, a professor at Purdue University who specializes in computer security.