Flight plan confirming Armstrong's first words on the moon to be auctioned

An original lunar flight plan which settles the debate about what astronaut Neil Armstrong meant to say when he first walked on the moon is expected to fetch as much as £55,000 at auction.

This 20 July, 1969 image shows Apollo 11 astronauts Neil Armstrong and Edwin E.'Buzz' Aldrin, Jr erecting the US flag at Tranquility Base during the First Lunar walk

The document carries an inscription by Armstrong that he had intended to say "one small step for A man, one giant leap for mankind".

But the indefinate article was accidentally omitted and the erroneous phrase "one small step for man ..." has gone down in history as one of the most memorable of the 20th Century.

Armstrong is said to have clarified his comment in writing while in quarantine after he returned from Apollo 11's epic journey in July 1969.

The document was authenticated by NASA press officer John McLeaish who dated it and wrote a declaration on the back.

McLeaish gave an interview in 2001 in which he said he had asked Armstrong what he had said and the astronaut responded: "Well I know what I was meant to say", before writing it down.

While the document clarifies the phrase itself, it does not clear up the matter of its provenance. Retired Nasa engineer Gary Peach claims that he coined the phrase because he feared that the landmark moment would pass without a suitably dramatic opening line.

After suggesting the words to his director ahead of the launch, the next he heard of it was when Mr Armstrong fluffed the line.

Armstrong insisted he came up with the phrase himself, but conceded he might have drawn on thoughts which were there "subliminally or in the background".

He also said in his autobiography that he never wrote the phrase down, but experts believe he must have forgotten about writing it directly after his return to earth.

The flight plan was a treasured keepsake for McLeaish until his death in 2007. It will be auctioned together with a letter written by his wife Patsy, who has said her husband was very proud of the gift from Armstrong.

The lot is being sold by a private collector on Tuesday April 13, at Bonhams in New York and, according to auction specialist Matthew Haley, has already stoked up international interest.

"These are arguably the most memorable words of the 20th century, inscribed by the man who spoke them," he said.

"McLeaish has written on the reverse: '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'."