A joint photographic portrait of Prince William and Prince Harry is to go on display for the first time.
The informal black and white picture of the royal brothers will be one of the star prints at a new exhibition that also features images of a host of celebrities.
Photographer Fergus Greer captured the image of the two princes dressed casually in jeans, chinos and shirts and looking relaxed.
The picture was taken in December 2006 at the Royal Military Academy, Sandhurst, in Surrey, where William was soon to graduate, following in the footsteps of younger brother Harry who had completed his course at the base earlier that year.
A Gallery spokeswoman said Clarence House commissioned Greer to take the double portrait which was to be used around the time of the Concert for Diana in August 2007, but never released.
Greer has photographed a number of celebrities from David Bowie and Julie Christie to Ray Winstone and Sir Michael Caine.
The royal brothers have had joint portraits taken before, most notably by celebrity photographer Mario Testino, who shot the now iconic informal portraits of their mother Diana, Princess of Wales just months before her death in 1997, and has also taken photographs for their father Prince Charles and the Duchess of Cornwall.
The image of the princes is one of 18 prints by 12 contemporary photographers acquired by London's National Portrait Gallery which will go on display from Monday until the autumn.
Also being exhibited will be a portrait of Tracey Emin by David Bailey and images of comics Ronnie Corbett and Lee Evans and actor Andy Serkis.