Two self-portraits by Francis Bacon are going on public display for the first time after being rediscovered in a private collection, before being sold.

Although experts knew the works by the late painter existed, they had no idea who had bought them.

Descendants of the original owner have decided to sell the paintings, which are expected to fetch up to £15m each.

The artworks are titled Self-Portrait 1975 and Three Studies for Self-Portrait (1980).

A Bacon painting featuring his friend and fellow artist Lucian Freud, became the most expensive artwork ever sold at auction when it fetched $142m (£89m) in New York in 2013.

Oliver Barker, Sotheby's senior international specialist in contemporary art, described the discovery of the portraits as "a pretty extraordinary collecting moment".

"(Art dealer) Marlborough Fine Art 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," he said.

"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."