"The second I saw it, my spine was shivering," said McCarthy, calling the piece a "Holy Grail" for collectors.
Both buyer and seller want to remain anonymous, and McCarthy did not disclose the- but experts note that a non-imperial Faberge egg sold at Christie's for $18.5 million in 2007.
Independent Faberge expert Geza von Habsburg said the egg is "absolutely genuine" and matches the one-line description found in records kept by Russia's Imperial Cabinet.
The egg, which contains a Vacheron Constantin watch, sits on a jeweled gold stand and was given by Alexander III to his wife Empress Maria Feodorovna in Easter 1887.
Only 50 of the imperial eggs were made for the royal family, and eight remained missing before the latest find, though only three of those are known to have survived the Russian revolution.
"I think it's pretty exciting," said Tatiana Zherebkina, spokeswoman for Faberge. "The experts seem to agree it's authentic and of imperial provenance - one of the eight missing eggs."
It will be on display at Wartski's London showroom April 14-17, the first time it will have been seen in public for 112 years.