Neil Armstrong, the first man to land on Moon, had fluffed his lines when he uttered the iconic phrase: “One small step for man, one giant leap for mankind”.

He had intended to say: “One small step for ‘A’ man, one giant leap for mankind.” But he missed out the ‘A’ to say “One small step for man, one giant leap for mankind”.

A small note scrawled on the original flight plan for the first moon landing in Mr. Armstrong’s own hand confirm that the astronaut fluffed his lines, The Sun reported.

It is believed that the phrase — perhaps the most famous of the 20th Century — written on the document is the only record of the words.

The revealing flight plan was inscribed and signed by Armstrong while in quarantine on his return to earth and handed to NASA press officer John McLeaish.

McLeaish authenticated the document while still with Mr. Armstrong by writing a declaration on the back of it and dating the item.

“This certifies that Neil Armstrong presented this signed page to me on August 9, 1969, while in quarantine following his mission as the first man on the moon,” McLeaish wrote.

Mr. Armstrong’s words when he put his foot on the moon on August 9, 1969, have been a subject of much debate.

Mr. Armstrong said in his autobiography he had never written the phrase down but experts believe he must have forgotten about writing it directly after his return to earth.

The flight plan with the words scribbled on it is now set to be auctioned by a private collector and is stoking up international interest.

The item is accompanied by a letter written by McLeaish’s wife Patsy, in which she has confirmed that her husband was very proud of the gift from Armstrong.

The auction is set to take place on April 13, at Bonhams auctioneers, in New York. The flight plan is expected to fetch £55,000.

“It is an extraordinary item, one of such historical importance,” said Andrew Currie, spokesman for Bonhams here.