A passenger plane-sized "void" has been discovered in the middle of the Great Pyramid of Egypt, where it has lain secret and untouched for 4,500 years, scientists has revealed.

The space is one of four cavities, along with the king and queen's chambers and Grand Gallery, now known to exist inside the giant monument constructed under pharaoh Khufu of ancient Egypt.

"It is big," said co-discoverer Mehdi Tayoubi of the ScanPyramids project.

The project has been exploring Khufu's pyramid since October 2015 with non-invasive technology using subatomic particle scans.

"It's the size of a 200-seater airplane, in the heart of the pyramid," Mr Tayoubi told AFP of the discovery, published in science journal Nature.

The Khufu's pyramid is the oldest and only surviving construction among the Seven Wonders of the Ancient World, and one of the largest buildings ever erected on Earth.

It towers over the Giza complex on Cairo's outskirts alongside smaller pyramids for kings Menkaure and Khafre and the Great Sphinx.

The cavity is the first major structure found inside the Great Pyramid since the 19th century, the research team said.

"There have been many theories about the existence of secret chambers inside the pyramid," said Mr Tayoubi. "But none have predicted anything this big."

The exact shape and size of the void is fuzzy - its purpose and possible contents a mystery.

But it is thought to be at least 30 metres long, and located above the Grand Gallery - a sloped corridor almost 50m long and 9m high which links Khufu's burial chamber at the pyramid's centre to a tunnel leading outside.

The monument - 139 metres high today, and 230 metres wide - was erected as a tomb for Khufu, also known as Cheops. To this day, nobody knows quite how it was built.

The pharaohs of ancient Egypt built these monumental tombs for themselves, complete with sarcophagus to hold their embalmed mummies, and stocked with everything they could require for the afterlife - food, clothing and jewellery.

Khufu's pyramid was plundered long before it was visited by modern archaeologists, and no relics remain from any of the known chambers.