The painting was once owned by US media tycoon William Hearst

The painting of a merchant's wife, Mrs Baldwin, is being offered for sale on the UK market next month from a private US collection.

Auctioneer Christie's says the portrait was believed to have been "lost" for more than 50 years.

In June a Reynolds portrait of Mary Wordsworth, Lady Kent, sold for more than �2.6m - five times the reserve.

That amount was the second-highest paid for a Reynolds at auction - his portrait of Omai went for �10.3m in 2001.

Reynolds' portrait of Mary Wordsworth fetched �2.6m

The latest sale involves a work completed in 1782 at the height of Reynolds' artistic creativity. It was a rarity in that Mrs Baldwin was his chosen subject rather than a commission.

It shows her seated on a divan in the exotic orientalist masquerade dress she wore at a court ball.

Born in Smyrna in 1763, she was married at a young age to George Baldwin, a rich merchant of Alexandria, although the marriage was unhappy.

She travelled widely and her beauty attracted many admirers including Emperor Joseph, the Prince of Wales and Dr Johnston.

The picture remained in Reynolds' possession and was sold in his posthumous studio sale in April 1796 where it was bought by the Swedish painter Carl Fredrik von Breda.

It later travelled across the Atlantic around 1892 when it was purchased from Christie's by R Hall McCormick, whose family owned the Chicago Tribune.

In 1920, media tycoon William Randolph Hearst acquired the painting and later sold it in New York in 1945.

It is being sold during British Art Week in London on 26 November.