The Savoy family are requesting the return of the jewels, which include tiaras, earrings, brooches and necklaces, studded with 6732 diamonds and 2000 pearls.

The jewels are said to have been the only part of the royal estate that were not confiscated by the Italian state after the monarchy was scrapped, an element that might help the descendants of the House of Savoy, including King Umberto II’s son, Vittorio Emanuele di Savoia, and three daughters, Maria Gabriella, Maria Pia and Maria Beatrice, win back possession.

The family is also hoping that a cryptic note delivered alongside the treasure chest, which said the “precious” crown jewels must be “entrusted to the custody of the central cashier” and “kept at the disposal of those who have right”, will help determine that the jewels belong to them and not the state.

The family is to sue Italy’s prime minister, the ministry of economy and the Bank of Italy for the return of the crown jewels, after more than 75 years, following the state’s rejection of a mediation attempt on Tuesday.

King Umberto II ruled for only 34 days before going into exile in Portugal, never setting foot in Italy again.

He died in Geneva, aged 78, in 1983.

Umberto’s father, King Victor Emmanuel III, reigned from 1900 until May 1946 when he abdicated in favour of his son after being discredited by his support for Mussolini during World War II.

He died in exile in Egypt, aged 78, in 1947.

Until now, the family had never taken any formal action to repossess the jewels over fears they might fuel a wave of resentment.

The Savoy family was represented at Tuesday’s meeting by Vittorio Emanuele’s 49-year-old son Emanuele Filiberto, who was born and raised in Switzerland.

The prince first set foot on Italian soil in 2002, after the Italian government lifted a law prohibiting male descendants of the House of Savoy from entering Italy.

“That the jewels remained locked in a Bank of Italy vault is ridiculous,” Emanuele Filiberto told Italian daily Corriere della Sera on Tuesday.

“If we get them back, we will have them exhibited.”