The likely temporary replacement for Gonzales is Solicitor General Paul Clement, who would take over until a permanent replacement is found, according to a senior administration official who spoke on condition of anonymity.
The Justice Department planned a news conference for 10:30 a.m. in Washington. President Bush was expected to discuss Gonzales' departure at his Crawford, Texas, ranch., before leaving on a trip to western states.
Two administration officials speaking on grounds of anonymity said that Gonzales had submitted a resignation letter last Friday. These officials declined to be identified because the formal announcement about Gonzales was still pending.
A longtime friend of Bush, who once considered him for appointment to the Supreme Court, Gonzales is the fourth high-ranking administration official to leave since November 2006. Donald H. Rumsfeld, an architect of the Iraq war, resigned as defense secretary one day after the November elections. Paul Wolfowitz agreed in May to step down as president of the World Bank after an ethics inquiry. And top Bush adviser Karl Rove earlier this month announced he was stepping down.
A frequent Democratic target, Gonzales could not satisfy critics who said he had lost credibility over the Justice Department's botched handling of warrantless wiretaps related to the threat of terrorism and the firings of several U.S. attorneys.
As attorney general and earlier as White House counsel, Gonzales pushed for expanded presidential powers, including the eavesdropping authority. He drafted controversial rules for military war tribunals and sought to limit the legal rights of detainees at Guantanamo Bay _ prompting lawsuits by civil libertarians who said the government was violating the Constitution in its pursuit of terrorists.
Bush and Gonzales had lunch over the weekend in advance of announcing his resignation. One said that Gonzales' resignation would take effect in two or three weeks.
Gonzales among about a dozen senior administration officials to resign amid a protracted congressional investigation into whatever role politics played in the dismissal of eight U.S. attorneys.
"It has been a long and difficult struggle but at last, the attorney general has done the right thing and stepped down," said Sen. Charles Schumer, D-N.Y., and one of the department's most vocal critics.
The flap over the fired prosecutors proved to be the final straw for Gonzales, whose truthfulness in testimony to Congress was drawn into question.
Lawmakers said the dismissals of the federal prosecutors appeared to be politically motivated, and some of the fired U.S. attorneys said they felt pressured to investigate Democrats before elections. Gonzales maintained that the dismissals were based the prosecutors' lackluster performance records.
Thousands of documents released by the Justice Department show a White House plot, hatched shortly after the 2004 elections, to replace U.S. attorneys. At one point, senior White House officials, including Rove, suggested replacing all 93 prosecutors. In December 2006, eight were ordered to resign.
In several House and Senate hearings into the firings, Gonzales and other Justice Department officials failed to fully explain the ousters without contradicting each other.
U.S. attorneys serve at the pleasure of the president, and can be removed. But congressional Democrats said politics played an unusually key role in the ouster of several prosecutors.
Bush repeatedly defended the firings of the prosecutors but acknowledged that he did not think Gonzales had done a good job of explaining it to Congress.