The famous picture has come to symbolise American resilience

The American flag raised by three firefighters over the wreckage of the World Trade Center in New York, one of the most enduring images of the 11 September attacks, has been lost.

During the clean-up operation, the flag was removed from the site, and is believed to have been flown on US ships involved in the military operations in Afghanistan, before being returned to New York in March.

But when it was given back to its owners, the flag was found to be bigger than the original.

Now city officials have launched a hunt for the flag, which has become a symbol of American triumph over adversity.

A spokesman for New York Mayor Michael Bloomberg said: "The mayor recognises the flag has become an iconic image of America's resilience and has asked the Fire Department to see if it can track the chain of custody."

Iconic image

According to the New York Times, city officials have traced the larger flag as far back as an appearance at the Yankee Stadium on 23 September.

They believe the original may have been accidentally switched or misplaced sometime between 11 September and then.

The flag in the photograph came from a luxury yacht, the Star of America, which was in a Hudson River marina near the World Trade Center on 11 September.

Madame Tussauds in New York has a waxwork of the photograph

Firefighter Dan McWilliams removed the flag from the yacht and walked back to Ground Zero, where he and two colleagues, George Johnson and Bill Eisengrein, raised it on a slanted pole.

The scene was captured by photographer Thomas Franklin, and distributed worldwide.

In the months after the terrorist attack, the flag travelled the world, going from the Yankee Stadium where it was signed by Rudolph Giuliani, then mayor of New York, to the Middle East, where it spent several months flying on ships involved in the war in Afghanistan.

It was then moved back to New York City Hall, where it was signed by Mayor Bloomberg and honoured in a solemn ceremony on 1 April.

'This isn't our flag'

The mistake was discovered a few weeks ago, when city officials returned the flag to Shirley Dreifus and her husband Spiros Kopelakis, who own the Star of America yacht.

It was at that point they discovered a switch must have taken place.

"We said `That isn't our flag,'" Ms Dreifus told the New York Times. "It's too big."

In fact, the flag that was returned to the couple measured 5 feet by 8 feet, while the actual flag in the photograph - attested to by the three firefighters in an affidavit - was 4 feet by 6 feet.