Sir Anthony van Dyck, Self-portrait 1640

Sir Anthony van Dyck's portrait of Katherine, Lady Stanhope, later Countess of Chesterfield c. 1635-6 will go on public display for the first time in over a century in Tate Britain's major spring exhibition, Van Dyck and Britain, opening on 18 February 2009. The painting was thought to be missing until it re-emerged at auction in New York in 2006. It will go on loan to Tate Britain from a private collection.

Katherine, Lady Stanhope, later Countess of Chesterfield c. 1635-6 is one of van Dyck's most beautiful portraits and is powerful in its simplicity. The artist painted Lady Stanhope (1609-1667) soon after the death of her first husband, Henry, Lord Stanhope. It seems that van Dyck had a personal involvement with Lady Stanhope, who divided her time between Kent, where her two children were being raised, and her house in fashionable Covent Garden.

The relationship between the artist and the sitter was recorded in a letter from Lord Conway to Lord Wentworth in 1636 describing van Dyck's “Gallantrye for ye love of that Lady”. The letter also describes a dispute between the pair about the price of the painting, raising questions about whether Lady Stanhope commissioned the portrait or if it was a lover's gift: “hee disputed with her about ye price of her picture and sent her word that if shee would not give ye price hee demanded hee would sell it to another yt would give more”.

The painting was once in the collection of the 1st Duke of Marlborough (1650-1722) before being sold to a New York dealer on 31 July 1886 as Lot No.229 on the third day of the 8th Duke of Marlborough's sale at Christies. In 1929 the portrait was sold to another private collector in the USA. Before its emergence in 2006 it was described in the Van Dyck catalogue raisonee as ‘whereabouts unknown'.

Rumours surrounded Lady Stanhope's love life. She was thought to be in love with Carew Raleigh and also apparently betrothed to Lord Cottingdon. In 1641 she married a Dutch nobleman, the Lord of Heenvliet (1597-1600) who had come to London to arrange the marriage of Charles I's nine year-old daughter, Princess Mary to William Prince of Orange. Lady Katherine and Lord Heenvliet moved to The Hague and were given senior appointments at the royal court. Katherine became Princess Mary's governess and gained her lifelong affection. Lord of Heenvliet died in 1660 at which point Katherine returned to England with the Princess when Charles II was restored to the throne. Katherine was created Countess of Chesterfield by the King. By October 1662, after Princess Mary's death, she had married her third husband, Daniel O'Neill, and in 1667 she died a very wealthy widow at the fine new house he had built for her in Belsize.

The painting will be displayed along with over 130 works by van Dyck and other British artists whose work he influenced, from the seventeenth to the twentieth centuries. The exhibition will include magnificent paintings from the Royal Collection, the National Trust and many private collections.