The No. 3 man at the Pentagon, Douglas J. Feith (search), undersecretary of defense for policy, is resigning his Pentagon position, FOX News learned Wednesday.

Feith's reasons for resigning are unclear, but Pentagon sources say the undersecretary will offer "family reasons" as his explanation. His last day will come at some point in the summer, the sources said.

Feith has not submitted a letter of resignation, but he verbally informed Defense Secretary Donald Rumsfeld of his intentions.

"He has had that discussion with me. I am hopeful he'll stay until we are able to find an appropriate successor, which we've not started looking for," Rumsfeld said in an evening press conference.

Pentagon spokesman Larry DiRita said Feith was weary of missing events in the lives of his children and wanted to spend more time with them.

"He feels good about this," DiRita said. "The up-before-the-kids and go to work, home-after-they-go-to-bed routine was getting old for him. He wants to participate more in their lives."

DiRita added that Feith had many tasks he wished to complete before his departure, including continued work on the Quadrennial Defense Review (search).

Feith, who follows Rumsfeld and Deputy Defense Secretary Paul Wolfowitz in civilian authority at the Pentagon, has spearheaded a number of major policy initiatives during his four years at the Pentagon, including the QDR, which aims to reposition American troops around the world and could mean a partial withdrawal of U.S. troops from South Korea and Germany.

But Feith has also taken a significant amount of heat over a number of issues, particularly since the beginning of the Iraq (search) war in March 2003. He has been criticized for his role in planning the Iraq war and for the administration's justifications for going to war.

Feith was in charge of the Pentagon's Office of Special Plans (search), which critics claim provided policymakers with uncorroborated prewar intelligence on Saddam Hussein's Iraq, including its alleged ties to the Al Qaeda terror network.

Pentagon officials have said the office was a small operation that provided fresh analysis on existing intelligence and was not in the intelligence-collection business.

Some groups have also pinned responsibility on Feith for cases of abuse of detainees at Guantanamo Bay, Cuba, in Iraq and Afghanistan.

Rumsfeld and Wolfowitz have both stated they plan to stay in their positions for the long haul.

FOX News' Bret Baier, Ian McCaleb and Nick Simeone and The Associated Press contributed to this report.