A 3,000-year-old sculpture of Tutankhamun was auctioned off by Christie's for nearly $8.4 million, despite Cairo calling on the UK government to stop the sale.
The organization did not share any information about the buyer.
Christie's decided to go ahead with the auction amid protests from Egypt's government and despite a small protest outside its London premises. Cairo has argued that the relic was smuggled out of Egypt and is still legally owned by the state.
"I believe that it was taken out of Egypt illegally," Mostafa Waziri, secretary general of Egypt's Supreme Council of Antiquities told the Reuters news agency. "(Christie's) has not presented any documents to prove otherwise."
International conventions prevent the sale of objects that are known to be stolen or illegally dug up.
Egyptian officials asked the UK Foreign Office and UNESCO to intervene, but such interventions are only made if there is clear evidence to dispute legitimate acquisition.
Christie's claimed it had performed "extensive due diligence" and "gone beyond what is required to assure legal title" of the piece.
According to the auction house, the sculpture came from the private Resandro Collection of ancient art.
It had allegedly been acquired from Munich art dealer Heinz Herzer in 1985, who bought it from Austrian dealer Joseph Messina in 1973-1974.