Two self-portraits by Francis Bacon are to cross the block at Sotheby's.

Each is valued at around £15m ($22.9m) ahead of a sale of contemporary art in London on July 1. They were recently rediscovered in a private collection.

Francis Bacon is among the world's most valuable artists

Oliver Barker of Sotheby's describes the unearthing of the works as "a pretty extraordinary collecting moment".

"Marlborough Fine Art [a London-based art dealer] kept a photographic archive and so both of these paintings appeared in a book on Bacon's self-portraits, but apart from being reproduced in books they've not been seen.

"We knew of the existence of the paintings but simply had no idea where they could be. The first time I saw these paintings it was such a wonderful awakening. They're both so luminous."

Francis Bacon holds the world record for the most valuable artwork sold at auction, after his Three Studies for a Portrait of Lucian Freud made $142.4m at Christie's in 2013.

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