Marden Henge has been almost destroyed by ploughing and no longer has any standing stones, but encloses an area of 15 hectares (37 acres).
A mound at the centre of the Wiltshire site still exists, which English Heritage archaeologists plan to spend six weeks delving into.
"We certainly hope that this excavation will bring more pieces of the puzzle to light," said archaeologist Jim Leary.
He added: "We are potentially looking at a much more intricate system of Neolithic ritual sites in this part of the world than we previously thought.
"Marden Henge deserves to be understood more partly because of its size, but also due to its proximity to the more famous stone circles at Avebury and Stonehenge."
Work on one of Britain's least understood ancient sites is to start.