US intelligence chief Dennis Blair has announced that he is stepping down in the first major shake-up of President Barack Obama's national security team.
Mr Blair's 16-month tenure as director of national intelligence (DNI) has been marked by infighting with the CIA and sharp criticism over the intelligence community's failure to prevent a botched Christmas Day attempt to blow up a US airliner.
James Clapper, under secretary of defence for intelligence, was expected to step into Mr Blair's role at least on an interim basis, according to a source close to the White House deliberations.
"It is with deep regret that I informed the president today that I will step down as Director of National Intelligence," Mr Blair said in a statement issued by his office. He said his resignation would become effective on May 28th.
Mr Obama praised Mr Blair's "remarkable record of service” to the United States.
"During his time as DNI, our intelligence community has performed admirably and effectively at a time of great challenges to our security, and I have valued his sense of purpose and patriotism," Mr Obama said in a statement.
The president is unlikely to leave the DNI job vacant for long at a time of heightened domestic security concerns following the Christmas Day aircraft bomb plot and a botched May 1st car-bombing in New York's Time Square.
US officials said the White House has already started interviewing "several strong candidates" to replace Mr Blair.
In addition to Mr Clapper, possible successors include John Hamre, who served as under secretary of defence from 1993 to 1997; Chuck Hagel, a former senator who co-chairs the Intelligence Advisory Board; National Counterterrorism Centre
director Michael Leiter; and Lee Hamilton, co-chairman of the September 11th Commission.
Mr Blair has operated largely out of the public eye, especially since the Christmas Day attack. To contain the political fallout following the December 25th and May 1st attempted bombings, the White House turned to top counterterrorism aide John Brennan, rather than Mr Blair, to lead the public relations effort.
Some current and former intelligence officials said Mr Blair's low profile undercut the DNI's standing and weakened his hold on the job. Another factor was Mr Blair's blunt style, which was seen by some as an asset but irked others in a White House that prizes staying on message.
Two sources close to the deliberations said Mr Obama had hoped to line up an immediate replacement for Mr Blair but was having trouble because of questions about the DNI's role. The DNI serves as the head of the intelligence community but has limited control over the CIA and its operations.
Officials say Mr Blair locked horns behind the scenes with CIA director Leon Panetta when he sought White House support to expand his office's authorities.
Many current and former intelligence officials trace the tensions back to 2004, when legislation overhauling the intelligence system created the office of the DNI without clearly delineating all of its powers.
Mr Obama took ultimate responsibility for lapses exposed by the Christmas Day plot, but the intelligence community's reputation suffered.
A Senate report released this week found that a wide range of intelligence and counterterrorism agencies missed chances to prevent the plot because of human and technical errors.