Archaeologists were “almost on the brink of giving up” when they made their “astonishing” discovery about Stonehenge, TV scientist Professor Alice Roberts has said.

Experts believe that “they may have recovered the true origins” of the ancient monument.

It is now thought that Stonehenge’s smaller bluestones originally formed an even older, long-lost monument in the Preseli Hills in Pembrokeshire, Wales.

Prof Roberts presents a BBC programme, Stonehenge: The Lost Circle Revealed, which followed archaeologists over years of digs and a decade of research.

Professor Mike Parker Pearson’s findings now show “that the original stones of Britain’s most iconic monument had a previous life”.

Speaking to the PA news agency, presenter Prof Roberts said the archaeologists “were always up against it”.

She said of their research and digging: “They had a terrible time. They’ve been doing this for about 10 years.

“They looked in lots of different places and didn’t find anything.

“They were almost on the brink of giving up and then they looked at this particular place called Waun Mawn.”

The site had previously been described as “doubtful and insignificant”.

But “they decided that they were going to dig anyway and just see if they can find anything, and they found these ghosts of stone holes…”

She added: “And they were exactly the same diameter as the outer circle at Stonehenge.

“It’s just unbelievable stuff, unbelievable archaeology.”

And Prof Roberts quipped: “On the one hand it shows that Stonehenge is a second-hand monument!

“It wasn’t set up there originally. It was set up in West Wales.”

The former Time Team star said the find was “the most exciting archaeology around Stonehenge that’s certainly happened during my lifetime”.

The presenter added: “It is incredibly exciting.

“When I first heard about it, I didn’t believe it at all.”

But she added of Prof Pearson: “He does seem to have found the origin of the earliest phase of Stonehenge.”

The stones may have been moved as part of a population movement out of West Wales.

The presenter said: “Of course there are questions about why these stones would have been brought from West Wales to Salisbury Plain … but ultimately it’s pre-history and we don’t know…

“Very often in pre-history we’re left asking those questions and I don’t think we’ll ever know because we just don’t have any written record.”

Archaeologists used 3D scanning techniques, traditional field archaeology, and laboratory analysis to discover when and where the stones for Stonehenge were quarried and where they first stood.

To end up with this astonishing breakthrough and to have a production company there filming the whole thing over the years is just amazing

She said the BBC documentary will feature the “highs and lows” before the discovery.

Prof Roberts said: “There were some days when they (the archaeologists) all went home tearing their hair out.

“So many programmes about discoveries on television are filmed after the discovery… (with) sometimes a bit of drama reconstruction to try and capture the excitement of the dig.

“But this production company… were so convinced that Mike Parker Pearson was going to find something exciting.

“So they’ve been following his story and him and his team on this trail for nearly 10 years. That’s in the programme.

“For a long time, it honestly didn’t look as though they were going to find anything…

“But to end up with this astonishing breakthrough and to have a production company there filming the whole thing over the years is just amazing.”

Stonehenge: The Lost Circle Revealed is on BBC Two at 9pm on Friday February 12.