The National Portrait Gallery has launched a public appeal to acquire a portrait of one of the "towering figures of British history", the Duke of Wellington.
More than 200 years after the Battle of Waterloo, the oil painting is being offered to the gallery, from a private collection, for £1.3 million.
The gallery said it has no other significant portrait of the military hero in its collection, which it described as "an omission of one of the most iconic and popular figures in British history".
It held an exhibition in 2015 to mark the bicentenary year of the Battle of Waterloo.
Sir Thomas Lawrence painted the portrait in 1829, but it remained unfinished when the British artist died in 1830.
Lawrence, described as the "Duke's definitive image-maker", created eight portraits of Arthur Wellesley, the 1st Duke of Wellington.
The gallery said the painting, commissioned when Wellington was prime minister, is one of only two "world-class" portraits of the Duke, who defeated the French under Napoleon Bonaparte at the Battle of Waterloo, ever likely to come up for sale.
The gallery needs to raise £300,000 in a public appeal by spring next year to buy the painting, saying it already has £1 million, comprised of its own funds and a £350,000 donation from the Art Fund.
The painting, pictured below, shows Wellington in civilian dress.
Broadcaster and historian Dan Snow said: "The Iron Duke is one of the towering figures of British history.
"He never lost a battle, reshaped Europe and dominated Britain until his death.
"His career and legacy are intimately involved with the development of the United Kingdom. Now, more than 200 years after his most famous victory at the Battle of Waterloo, it's time we helped the National Portrait Gallery win the day."
The gallery said that, while it does have portraits of the Duke in its primary collection, none of them "do justice ... in either historical or art historical terms", and that they do not depict Wellington at the peak of his participation in public and political life.
Dr Nicholas Cullinan, director of the National Portrait Gallery, said: "We have been searching for a portrait that can do justice to this iconic British hero since 1856.
"The lack of a suitable depiction of the Duke of Wellington has long been identified as the biggest gap in our collection. If we can raise the funds, this remarkable painting by Sir Thomas Lawrence will be on permanent display and free for over two million visitors to enjoy each year."
Dr Lucy Peltz, senior curator at the gallery, said: "This is a compelling portrait of one of the most famous figures in early 19th century Britain.
"Lawrence was a superlative portrait painter with the flair and talent to capture surface glamour and deeper currents. This unfinished portrait is shot with psychological insight."