Her hair severely parted, Constanze Weber Mozart looks unsmilingly away from the camera. She appears to be staring at her feet. Next to her is Max Keller, a Swiss composer and old family friend, surrounded by his daughters and the rest of his family. In the background is a cottage with two garden-facing windows.

The newly discovered black and white image is the only photograph ever taken of Constanze Mozart, the widow of the Austrian composer and genius Wolfgang Amadeus Mozart.

The previously unknown print was discovered in archives in the southern German town of Altötting, local authorities said on Friday, and has now been authenticated as including Mrs Mozart, on the far left.

The long-lost photograph was taken in October 1840, when Constanze Weber was 78 years old, at Max Keller's home. The Altötting state archive said it was the only time in her life that she was photographed.

The picture is one of the earliest examples of photography in Bavaria, it said. Daguerrotype photography was first practised in the southern German state around that time.

The photo of Mozart's widow was not the original but a copy made in the second half of the 19th century, officials added.

Mozart married Constanze Weber in 1782. The couple had six children together, only two of whom survived infancy. After Mozart's death in 1791 Constanze, then 29, found herself having to bring up her young family on her own. She later married a Danish diplomat, Georg Nissen, and they regularly visited Keller at his home in Altötting.

The couple quickly realised Mozart's potential as a lucrative source of income, with Nissen publishing a Mozart biography and ensuring the composer's posthumous reputation was kept alive throughout the 19th century.