A 5,000-year-old monument that was aligned with the summer and winter solstices and may have served as a prototype for the later solar alignment at Stonehenge has been discovered close to the famous neolithic site, in what archaeologists have described as a “once in a lifetime” find.

The structure at Bulford, 5km (3 miles) from the world heritage site in Wiltshire, has been carbon dated to around 3000BC, the same time as the earliest phase of construction at Stonehenge and 500 years before its huge trilithon stones were carefully placed to line up with the midsummer and midwinter sun.

It is the earliest solstice-aligned structure in the Wiltshire landscape and one of the very first in Britain, according to experts. The archaeologist Phil Harding, who led the dig on behalf of Wessex Archaeology before the construction of new Ministry of Defence housing, said the discovery was “one of the greatest finds of my career”.

Harding nearly didn’t spot it at all, however. Unlike Stonehenge, whose immense solstice-aligned sarsen boulders are still standing 4,500 years later, the Bulford monument consisted of two wooden poles 120 metres apart, which had left only two large post pits in the ground surrounded by a jumble of smaller rubbish pits.

Harding, a former presenter on Channel 4’s Time Team, said at first, he and his colleagues had not recognised their discovery. It was only in later analysis of the site plan, when he drew a line with pencil and ruler between the anomalous pair of larger postholes, that he recognised the solstice alignment. “The thing that struck me as soon as I saw that was that [the line was] about 50 degrees off the direct north, which was pretty much the line of the midsummer sunrise. And so I got really, really excited about that.”

Further work by Fabio Silva, a “skyscape archaeologist” and expert in ancient astronomical mapping, confirmed that the two wooden poles very accurately lined up with the midsummer sunrise and midwinter sunset in 2950BC, the date of the structure according to extensive radiocarbon analysis.

Based on the 1-metre depth of the post pits, the team believe the wooden poles stood 3-4 metres high and would have aligned in a “gunsight” with the solstice sunrise and sunset. A smaller pit, also aligned with the poles, contained a rare disc-shaped flint knife, which the archaeologists say may have been shaped to represent the sun.

“What we’re seeing here is the religion of the stone age made manifest in the ground,” said Matt Leivers, the senior research manager at Wessex Archaeology. “Obviously we have no understanding of precisely what any of it meant, but the fact that time and again, over thousands of years, people are coming back to [the Stonehenge landscape] to build and rebuild and mark and remark this set of substantial events – it gives us an indication that this is religion. This is how they are understanding their place in the cosmos, how the universe works, what their deities are.

“We don’t know what the sun meant to them. We don’t know whether they personified it as a deity. But the amount of effort that’s directed toward marking it and its movements leaves us in no doubt at all that this is a major religious event that’s inscribed over the whole landscape over millennia.”

Leivers said it was “inconceiveable” that those commemorating the solstices at Bulford would have been unaware of those doing the same at Stonehenge – and may in fact have been the same people. He said: “If you had a time machine and went back, I wouldn’t be at all surprised if what we have found is one of the campsites of the builders of the first phase of Stonehenge. I think that’s entirely plausible.”

“Sites like this come along once in a lifetime, sometimes they don’t come along at all,” said Harding. “It doesn’t matter whether you are a resident of Wiltshire or a resident of the Earth – everybody knows about Stonehenge. And to be able to contribute something to expanding our knowledge of Stonehenge is an incredible privilege.”