Andrew Card, the White House Chief of Staff who whispered in President Bush's ear to warn him that America had come under attack on September 11, has resigned.

Mr Card, 58, is to be replaced from April 14 by Josh Bolten, 50, currently director of the Office for Budget and Management.

"Earlier this month, Andy Card came to me and raised the possibility of stepping down as Chief of Staff," Mr Bush told reporters at the Oval Office. "After five and a half years, he thought it might be time to return to private life. This past weekend, I accepted Andy’s resignation."

He added: "I have relied on Andy’s wise counsel, his calm in crisis, his absolute integrity and his tireless commitment to public service."

The move comes amid a sharp decline in Mr Bush’s approval ratings and calls from Republicans for the President to inject fresh blood into his team.

Mr Card has been Chief of Staff since Mr Bush was first elected in 2000 and was reappointed after his election in 2004. In that role he has acted as Mr Bush's gatekeeper, assisting with key policy decisions and managed the White House on a day-to-day basis.

A longtime Republican stalwart and Bush family loyalist, Mr Card served as a deputy chief of staff and also as the US Secretary of Transportation under the administration of the first President Bush. He also served as a special White House assistant during Ronald Reagan’s administration.

During the Clinton years, Mr Card led the American Automobile Manufacturers' Association and then worked as a vice-president at General Motors.

If Mr Card had stayed on until September he would have become the longest-serving chief of staff ever, surpassing the record of Sherman Adams, who ran the White House under Dwight Eisenhower for five years and nine months.

But Mr Bush’s ratings have tumbled over concerns about his Iraq policy, and his perceived lacklustre response to Hurricane Katrina. A White House scandal over the leaking of a CIA agent’s identity to the media has also taken its toll.

Mr Card will be best remembered for breaking the news to Mr Bush on September 11, 2001, that a second plane had flown into one of the Twin Towers in Manhattan, bending over to whisper in his ear while Mr Bush reading with children at an elementary school in Sarasota, Florida.

Mr Bush had already been told about the first plane hitting the tower before entering the classroom, but at that point it was thought to have involved a light aircraft and the event went ahead. When the second plane hit, and the gravity of the situation became clear, Mr Card decided to take the rare step of interrupting the president.

Walking through the classroom he approached Mr Bush and said: "A second plane hit the second tower. America is under attack."

His replacement, Mr Bolten, rides a Harley-Davidson and is part of a White House rock band. He became director of the Office of Management and Budget after serving as deputy chief of staff in the White House from 2001 to 2003.

He is the son of a CIA officer and is known in the White House as a quietly effective manager. "I’m honoured Josh has agreed to serve," Mr Bush said.

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