The work was found by Dorothea Link, a musicologist, who has been teaching at Dalhousie University, in Halifax, Nova Scotia, for the last year. She came across the passage, which is scored for strings and soprano, in the Austrian National Library, in Vienna.
Stanley Sadie, editor of The New Grove Dictionary of Music and Musicians, said it was almost definitely by Mozart.
Mozart was a friend of Martin y Soler who had written an opera called Il Burbero di Buon Cuore. But it was customary to rewrite arias for singers if they wanted. Mr Sadie said it was known that Mozart had rewritten the aria for Louise Villenueve. "Mozart's original score for `Vado Ma Dove' is in Berlin Library and it was thought that was all he had written for this opera. But Ms Link found a small manuscript interleaved with the opera score," he said. "Although we can never have 100 per cent proof, it is extremely likely that it is him."