Growing acrimony over the failures of the Hurricane Katrina emergency relief effort claimed its first political victim on Friday when Michael Brown, the top US official, was relieved of frontline responsibility.

Mr Brown, who as head of the Federal Emergency Management Agency has been heavily criticised for the slow response to the disaster, was replaced immediately by Thad Allen, a Coast Guard vice-admiral.

The decision to switch a political appointee with a military professional was seen as a White House attempt to deflect public anger at the handling of the crisis.

Mr Brown was recalled to Washington to “direct operations nationally” and address the threat of further hurricanes, Michael Chertoff, head of the Department of Homeland Security, announced. Although Mr Chertoff insisted that Mr Brown had “done everything he could to co-ordinate the federal response to the unprecedented challenge”, his credentials and performance have come under intense scrutiny in recent days.

His suitability for the post has been widely queried, with critics pointing to his previous career as a lawyer and commissioner for the International Arabian Horse Association, a breeders and horse show organisation. Mr Brown's appointment at Fema, they added, came through Joe Allbaugh, the former head of the agency and a college friend.

His command of the task was challenged last week when he said he did not know that 20,000 evacuees were stranded in the New Orleans Convention Center with little food or water even though their plight had been widely reported.

The pressure on Mr Brown was increased by a Time magazine report that alleged he had embellished his resume, claiming for example that from 1975 to 1978 he had overseen the emergency services division in Edmond, Oklahoma. The city said he had been merely an “assistant to the city manager” at that time.

Democrats have seized on Fema's shortcomings as a way to pin blame on the White House. Mr Bush's approval ratings and reputation for leadership have both suffered in the wake of Hurricane Katrina.

A Zogby poll on Thursday said just 41 per cent approved of his job as president, an all-time low for that poll, and 36 per cent approved his handling of Hurricane Katrina. In a separate CBS poll, just 48 per cent said they saw Mr Bush as a strong leader, down from the 83 per cent levels he enjoyed after September 11.

Nancy Pelosi, House minority leader, and Dick Durbin, senator for Illinois, this week called for Mr Brown's resignation and said President George W. Bush was “oblivious, in denial and dangerous”.

Hillary Clinton, senator for New York, said she would “never have appointed such a person”.

Mr Chertoff said: “We are now in a period when we are moving from immediate emergency response to the next phase of operations. The vice admiral is doing an excellent job and has my full support in the work ahead.”

This week Mr Bush secured approval from the House for $51.8bn in additional emergency relief for the region and promised further aid would be available