Archaeologists are stunned to have found a prehistoric square monument beneath the world-famous Avebury stone circle.

Academics from the University of Leicester and University of Southampton used soil resistance surveys and radar to investigate the famous stone circles just 11 miles south of Swindon.

There are three stone circles at Avebury, built over several hundred years in the third millennium BC – in what archaeologists call the late Neolithic era.

The UNESCO World Heritage Site, which is now owned and managed by the National Trust, includes the largest stone circle in Europe, originally comprising around 100 massive stones and stretching 330m in diameter.

In the 1930s excavations by gentleman archaeologist and marmalade magnate Alexander Keiller uncovered an unusual angular setting of small standing stones. They stood near a single huge upright stone known since the 18th century as the Obelisk.

The excavations were interrupted by the outbreak of World War Two and it wasn’t until decades later that archaeologists returned to investigate the curious angle.

Dr Joshua Pollard from the University of Southampton said: “Our careful programme of geophysical survey has finally completed the work begun by Keiller. It has shown the line of stones he identified was one side of a square of megaliths about 30m across and enclosing the Obelisk.

“Megalithic circles are well known from the time when Avebury was built during the late Neolithic era, but square megalithic settings of this scale and complexity are unheard of.”

Archaeologists say that the square could be one of the earliest structures on the site, and might have been built to commemorate an earlier structure that could once have housed Avebury’s first settlers.

Today’s visitors to Avebury can only see one corner of this once massive stone square – a handful of stones uncovered by Keiller eight decades ago.

Dr Nick Snashall, National Trust archaeologist at Avebury, said: “This discovery has been almost eighty years in the making but it’s been well worth waiting for.

“The completion of the work first started by Keiller in the 1930s has revealed an entirely new type of monument at the heart of the world’s largest prehistoric stone circle, using techniques he never dreamt of.

“It goes to show how much more is still to be revealed at Avebury if we ask the right questions.”

The work was funded by the Swindon-based Arts and Humanities Research Council. Archaeologists are writing up their findings for an academic journal.