A "lost" portrait by Gainsborough of his daughter is to go on public display for the first time in 131 years , after being identified thanks to a letter in Country Life magazine,

The "sensationally beautiful" oil painting, entitled Margaret Gainsborough, The Artist's Daughter, Playing A Cittern, was still in Gainsborough's possession when he died and stayed with his descendants until the 1870s.

But after being sold into a private collection in 1908, its whereabouts were not recorded.

Curators at the National Portrait Gallery, who are staging an exhibition of the artist's work, spent five years searching for the unfinished work before writing to Country Life magazine just in case any readers could help them.

The owners, who have asked not to be identified, recognised the "touchingly personal" painting, which has not been on public display since 1887 and will now go on show at the gallery in London.

Curators have made last-minute changes to the show, Gainsborough's Family Album, before it opens, to make room for the work.

The exhibition will focus on the works Gainsborough created "for love, rather than money" and track his life from "relative obscurity to being one of the leading lights of the British art world".

Lucy Peltz, curator, said hunting down paintings for the show involved some "detective" work.