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.

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.