The dictator allegedly gave the diaries, documents and letters to a senior aide just a few days before he was captured and executed by partisans as he tried to flee to Switzerland at the end of the war.
The papers - contained in a padlocked, damp-proof zinc suitcase - were reportedly buried in the Spluga Valley, north of Lake Como, near where Mussolini and his lover were caught, killed and strung up from a petrol station.
The claim has been made by the son of Guglielmo Della Morte, Mussolini's consul-general to Germany during the war.
Rocco Della Morte claims that Il Duce gave his father the suitcase, which bore the initials B.M., in April 1945, shortly before his capture.
He believes the case may also contain important state papers which Mussolini hoped to use in negotiating his surrender with the British and Americans.
"It's perfectly feasible that Mussolini bundled up important papers and consigned them to a secure location," said Christopher Duggan, an authority on Mussolini from Reading University.
"All the leading fascists were in a panic and desperate to make arrangements with the Allies. "They were thinking about what documents might help them when they faced war crimes trials."
Mr Della Morte claims that his father told him about the location of the diaries in 1954, when he turned 18, and swore him to secrecy, ordering that the location not be revealed until 2025.
He said he decided to reveal the story now because he is 74 and the information would otherwise die with him.
"I have checked up on the suitcase and I've verified that it is still there," he said.
He said he had made arrangements for the suitcase to be recovered and opened, and for its contents to be published.
A leading Italian historian, Marino Vigano, said he thought it more likely that any "lost" diaries and documents belonging to Mussolini ended up either in the imperial archives of Japan, Italy's ally during the war, or in secret files in the Vatican relating to the wartime papacy of Pius XII, which are not expected to be opened for another five or six years.
By Nick Squires in Rome
