It was humankind's crowning achievement, with millions around the world glued to their television sets as US astronaut Neil Armstrong took the first steps on the moon 40 years ago.

But in the scientific equivalent of recording an old episode of EastEnders over the prized video of your daughter's wedding day, Nasa probably taped over its only high-resolution images of the first moon walk with electronic data from a satellite or a later manned space mission, officials said today.

It means that the familiar grainy and ghosting images of Armstrong's "giant leap for mankind" are all that remain from the mission, though the space agency has managed to digitally restore the footage into new broadcast-quality pictures that it released today.

"I don't think anyone in the Nasa organisation did anything wrong. It slipped through the cracks and nobody's happy about it," said Dick Nafzger, one of the last Apollo-era video engineers still working for the agency at Maryland's Goddard Space Flight Centre.

In a technological feat that rivalled even putting Armstrong and his shipmate Buzz Aldrin on the lunar surface, and one that has been largely overlooked since, a team of Nasa engineers and contractors fed live video from the moon, via a series of relay stations in Australia and the US, to homes around the world.

While Armstrong, Aldrin and Apollo 11 pilot Michael Collins trained for the mission, Nafzger and his partners were tasked with working out how to broadcast live from 240,000 miles away.

The images of Armstrong and Aldrin stepping on to the lunar surface and planting the US flag in the grey dust were seen by an estimated 600 million people. The tape recordings, taken for backup, were an afterthought, Nafzger told reporters in Washington today.

"We all wish that somebody had said 'those tapes are special, let's pull them aside'," he said.

Instead, their loss apparently went unnoticed for 35 years, until 2004, when an archive in Australia alerted Nasa that it believed it had found the lost tapes from the Apollo 11 mission. It shipped the tapes to Goddard, where Nasa maintains what officials say is the only machine in the world capable of reading the old tape technology. The first tapes did not have moon footage, but touched off a massive search for those that did in archives stored in dusty basements across the world.

Nasa believed the original tapes might contain digital data sent from the moon that could be converted into much sharper pictures of the landing than those broadcast on the day, which were taken by a television camera pointed at a giant wall monitor at mission control in Houston - effectively a copy of a copy.

But a standard Nasa money-saving measure in those days was to reuse the 14-inch tape reels after several years in storage. Agency officials ultimately concluded that the original Apollo 11 tapes were buried among an estimated 350,000 that were recycled in the 1970s and 1980s and the data was lost for ever.

The newly released footage was taken from four copies, including one in a CBS television archive. It is undergoing restoration by a firm that specialises in cleaning up old Hollywood movies.

"I don't believe that the tapes exist today at all," Stan Lebar, the designer of the original lunar camera, told America's National Public Radio. "It was a hard thing to accept. But there was just an overwhelming amount of evidence that led us to believe that they just don't exist any more."