"It may be intact, and inside there is probably a wooden sarcophagus and maybe even a mummy," said Dr Hawass. "We will start excavating next week."
The pyramid of Queen Ankh-sn-Pepi, wife of King Pepi I, lies in Sakkara, an ancient royal cemetery just 32 kilometres (20 miles) south of Cairo.
Archaeologists, led by Jean Leclant, found a stone in the queen's burial chamber bearing special prayers to protect the dead and ensure sustenance in the afterlife. Until this discovery, such texts had only been found in pyramids of kings. Why they were placed in the Queen's chamber remains a mystery.
The finding was one of several announced at the Eighth International Congress of Egyptologists that has drawn some 1,500 archaeologists to Cairo.
Mummy hope
In another discovery, Egyptian archaeologists said they had found a painted tomb in the Western Desert. It was built by people from a 600 BC culture that exported wine to the Nile valley.
Leading Egyptian archaeologist Zahi Hawass said he saw a burial chamber containing a four metre (13 feet) long stone coffin through a hole in a wall of the tomb.
"It may be intact, and inside there is probably a wooden sarcophagus and maybe even a mummy," said Dr Hawass. "We will start excavating next week."
The tomb is in the Valley of the Golden Mummies, 344 km (215 miles) south-west of Cairo. The area made headlines last year when 105 mummies were found in a vast cemetery of Greco-Roman tombs.
Archaeologists made this latest discovery while re-excavating three other similar tombs. Ten houses built above the newly-discovered tomb were removed and Dr Hawass said the government will relocate the homes and compensate the families.
Mysterious chambers
In a third discovery announced at the conference, a joint expedition of Egyptian and French archaeologists said they had found two additional chambers and a corridor in the collapsed pyramid of Maidum. The pyramid, which lies 90 km (56 miles) south of Cairo, dates back to 2600 BC.
Dr Gaballa said the new chambers had only been viewed through an endoscope, a 30 m (99 ft) long flexible tube inserted through joints in the stones.
He said the purpose of the hidden chambers is not yet known, but they may have been built to lessen the weight on the burial chambers below.