LONDON -- A previously unknown portrait of Mozart has been found, and could be the most important portrait of the renowned composer ever to come to light, a British academic said Friday.

The discovery -- which could be worth several million dollars -- was owned by the family of Johann Lorenz Hagenauer, a close friend of the Mozart family in Salzburg.

"This is arguably the most important Mozart portrait to be discovered since the composer's death in 1791 and only the fourth known authentic portrait of him from his Vienna years," said Professor Cliff Eisen.

Eisen, from King's College London, undertook extensive research to authenticate the portrait, which is painted in oils and measures 19 inches (48 centimeters) by 14 inches, showing the composer in profile in a red jacket.

After studying letters from Mozart and his family, archival documents and estate auction records, he believes it was probably painted in about 1783 by Joseph Hickel, a painter to the Imperial Court of Austria.

Family tradition has it that it was a gift to Mozart in return for dedicating a composition to a member of the Hickel family.

An American collector bought the portrait in 2005 and was unaware of its significance until the Hagenauer connection was established.

Professor Simon Keefe, James Rossiter chair of music at Sheffield University, in northern England, said Eisen had made a "persuasive case" about its provenance and the find was "inherently significant".

"Needless to say, it will encourage us to think afresh about Mozart's appearance," he said in a statement.

"Furthermore, it should soon as of right join the other two most famous pictures of the adult Mozart -- an unfinished painting by Mozart's brother-in-law Joseph Lange and a posthumous one by Barbara Kraft -- as the defining image of the composer in the public consciousness."

Born in Salzburg in 1756, Wolfgang Amadeus Mozart took up residence in Vienna in 1781 where he composed his most famous works. He is generally considered to be one of the world's greatest composers.