The man chosen by President George W Bush to take over as the new US homeland security secretary has withdrawn his nomination.
Mr Kerik was nominated by President Bush just a week ago
Bernard Kerik took the decision after uncovering tax and immigration problems relating to his employment of a nanny.
A White House spokeswoman said Mr Bush respected Mr Kerik's personal decision.
The former New York City police chief, was chosen a week ago to head the department created after the 11 September 2001 attacks.
His role in their aftermath transformed him into a symbol of strength.
Rudolph Giuliani, the former New York mayor who supported the appointment, said Mr Kerik had little option but to step aside.
"Bernie did not want to put the president through a difficult confirmation process where the odds were, because of this issue, he wouldn't get confirmed," Mr Giuliani told reporters in New York.
"I uncovered information that now leads me to question the
immigration status of a person who had been in my employ as a
housekeeper and nanny," Mr Kerik said in a statement.
"It has also been brought to my attention that for a period of time, during such employment, required tax payments and related filings had not been made."
Mr Kerik added that he feared the disclosure would "only serve
as a significant and unnecessary distraction to the vital
efforts of the Department of Homeland Security," in excerpts quoted by Reuters news agency.
1986: Joined NYPD as beat officer
1991: Awarded medal for valour after shoot-out
1997: Rose to rank of commissioner
2001: Oversaw department during 9/11
"I did so on my own. I thought it was the right thing to do," Mr Kerik said.
Ahead of formally taking office, Mr Kerik faced intense scrutiny both from the FBI and from the media.
News organisations raised questions about his links to the stun-gun company, Taser.
Mr Kerik, who sat on the board of Taser, earned $6.2m from stock options he received from the company, which had done highly profitable business with the department Mr Kerik was to head, according to FBI sources quoted by the Associated Press.
He was also fined two years ago for using a police sergeant and two detectives to research his autobiography, the New York Times has reported.
The White House has said it will move as quickly as it can to name someone else to fill this nomination but the decision means President Bush will have to seek a new candidate to run the
sprawling bureaucracy of more than 180,000 employees.
Mr Kerik was thrust into the public spotlight in his position as New York police commissioner after the 11 September 2001 attacks.
He supervised the police department's response and was
often seen at the side of then-Mayor Rudolph Giuliani.
In 2003 he took on a temporary assignment in Iraq to help rebuild
the country's police force, and played a key role immediately following the cessation of major hostilities.
He also served as interior minister at the Coalition Provisional Authority, the body set up by the US Department of Defence to run Iraq following the fall of Saddam Hussein.
The BBC's Daniel Lak in Washington says that Mr Kerik's strong physical and symbolic presence mean it will prove difficult for Mr Bush to find someone with the same high media profile to step into his shoes.
Among possible candidates is the president's homeland security adviser, Frances Townsend.