It is one of the defining images of September 11. White House Chief of Staff Andy Card leans in to tell President Bush that a second plane has hit the World Trade Center in New York.

Speaking to the BBC's Today programme on Saturday morning, Card recounted the moment he learned of the attacks and what he told the President.

Bush was sat reading the book "The Pet Goat" in a classroom in Florida with a group of schoolchildren at the time.

Card said that he had initially been told by an aide that one of the Twin Towers had been hit by a small plane by accident. He said the collective reaction was "oh what a horrible accident, the pilot must have had a heart attack".

But he said he was soon told that another plane had hit the other tower and that it was not a coincidence or an accident. Card said his mind "flashed to the fear experienced by the passengers" as he prepared himself to tell the President what had happened.

"I leaned over and whispered 'a second plane hit the second tower, America is under attack'," he said. "That's all i said to him."

"I stood back so he couldn't talk to me and ask me a question."

Card recalled how the president stayed seated "shaking his head a bit".

Bush attracted much ridicule for appearing indecisive by failing to react to the news. Rather than get up and leave, the President sat with the children in the classroom for another seven minutes.

But Card said it was right that Bush did rush out of the room straight away.

"I thought he reacted well, he did nothing to introduce fear to those very young students or demonstrate fear to the media that would have satisfied the terrorists," he said.

And Card revealed that he had Tony Blair to thank for the extra security protection he was given after the image was flashed around the world.

He said the then British prime minister, on a visit to the United States in the days after the attacks, had joked Card was a "marked man" as the photograph would become "target practice for every terrorist in the world".

Card said the secret service, who protect the president and other important political figures, told him later that day: "we own you 24/7 starting now".