Expedition leader Mehdi Tayoubi, president of the Heritage Innovation Preservation Institute, Paris, said: "We were very surprised to see a big anomaly. We were expecting to detect the King's chamber and grand gallery but we were surprised to find a big void and we think what we have is very important.
"What we are sure if it is there, and it is impressive, It is the same height as the Statue of the Liberty.
"It could be a second grand gallery, it could be a chamber. We are working with archaeologists to compare it to other chambers. The void is there, it's very big. It was not expected by any kind of theory.
"We are thinking about sending in a very innovative robot that can get through a small hole."
The Great Pyramid of Giza which was built for Khufu, or Cheops, the second pharaoh of the 4th dynasty who reigned between 2589 and 2566BC. His tomb is 140m high and 230m wide, the largest pyramid ever built, and one of the Seven Wonders of the Ancient World.
However, experts are divided about how the Egyptians moved the massive stones, some of which weigh 80 tonnes, from Aswan, more than 800km away. Some theories suggest elaborate cranes and pulleys, wooden rail tracks, spiral outer ramps, or even floatation chambers.
The new void is 70m from ground level and scans suggest it may be sloped at a similar angle to the Grand Gallery below.
Dr Kate Spence, senior lecturer in Archaeology and Egyptology at the University of Cambridge, said the incline could be the key to the purpose of the chasm, and help solve the mystery of how the Egyptians got their huge stones inside.
"This finding is very exciting, but not because it is some kind of secret chamber," she said.
"I think it is an inclined ramp that was used to transport huge blocks into the centre of the pyramid and then sealed off by the builders. The orientation leads up to the huge granite roof struts at the top of the relieving chamber.
"Although that might not sound as exciting as a secret burial chamber, in fact, this would be the first evidence of the use of a ramp inside, which gives us an important insight into how those huge bricks were put into position and how pyramids were constructed."
Professor Hany Helal, of the University of Cairo, said the orientation resembles a corridor that was found hidden behind the North Face.
"It's too early to go and conclude what is this void. Was it deliberately hidden away? This structure is not accessible. This void was hidden since the construction of the pyramid.
"While there is currently no information about the role of this void, these findings show how modern particle physics can shed new light on the world's archaeological heritage."
Jean-Baptiste Mouret of ISIR in France, said the team would be releasing details next week of a small flying robot that could fit into a 2.5cm-wide hole to explore the void. Any drilling into the pyramid would require permission from the Egyptian Ministry of Antiquities.
Once the mystery of the void is solved, archaeologists hope to use similar technology to find the burial place of Queen Nefertiti, the wife of King Akhenaten. Some Egyptologists think she could be buried in a secret chamber in Tutankhamun's tomb.
The research was published in the journal Nature.
