A long-lost collaborative effort between composers and putative rivals Wolfgang Amadeus Mozart and Antonio Salieri has been found at a Czech museum.

"It's a really valuable work... long thought to have been lost," Czech National Museum spokeswoman Sarka Dockalova said, adding that staff discovered it in the reserve collection.

"It's a joint composition by Mozart and Salieri — a libretto by Lorenzo Da Ponte put to music."

Ms Dockalova said it would be performed on Tuesday at a press conference in Prague.

The discovery is especially interesting in light of a legend discounted by historians — Italy's Salieri was said to have fatally poisoned Mozart out of jealousy over the Austrian wunderkind's talent.

First appearing in Alexander Pushkin's 19th century poetic drama Mozart and Salieri, the rumour was later featured in the play and in 1984 film Amadeus, which historians say grossly exaggerated Salieri's rivalry with Mozart.

The film was shot in the Czech capital Prague, where Mozart spent considerable time in the 18th century as it was then part of the Austrian Empire.

AFP