The supposed mistake has dogged Armstrong for nearly four decades

On tapes of the Moon landings, he appears to drop the "a" from the famous quote: "That's one small step for a man, one giant leap for mankind."

But new analysis of the tapes has proved Mr Armstrong right after all.

Computer programmer Peter Shann Ford used audio analysis software to show that the missing "a" was blotted out by transmission static.

Even Mr Armstrong has never made up his mind about whether he got the quote right.

"It doesn't sound like there was time for the word to be there," he told author James Hansen in his 2005 biography First Man.

I find the technology interesting and useful - I also find his conclusion persuasive

Neil Armstrong

"So I would hope that history would grant me leeway for dropping the syllable and understand that it was certainly intended, even if it wasn't said - although it might actually have been."

But an analysis of the audio files downloaded from Nasa's website using GoldWave, a $45 (£24) audio editing program, indicates that the word was spoken but not recorded by Mr Armstrong's microphone before being transmitted to the 500 million people watching the Apollo 11 mission.

Ford said that Mr Armstrong completed the whole phrase "one small step for a man" too quickly to pick up every syllable he said.

But the audio analysis was able to find the signature of the missing word, he said.

Technology backed

In a statement, Mr Armstrong supported the evidence presented.

"I have reviewed the data and Peter Ford's analysis of it, and I find the technology interesting and useful," he said.

"I also find his conclusion persuasive. Persuasive is the appropriate word."

More than half a billion people watched the moon landings live

Mr Armstrong says that he came up with the phrase in the hours between the touchdown of the lunar module and his first steps onto the Moon's surface.

But without the missing "a", the meaning of the quote is lost. In effect, the line means: "That's one small step for mankind, one giant leap for mankind."

Former Nasa historian Roger Launius told the Houston Chronicle that he supported Ford's conclusions.

"It's nice to know that what he thought he said, he actually did say, and that because of the nature of the electronic and the communications systems of the time, it just did not get through."