French gendarmes say workers paid to demolish an uninhabited house in Brittany made an unexpected discovery in the cellar — 600 gold coins.

The Pont-Aven gendarmerie said on Monday (local time) that after rattling a mysterious, shell-shaped container, the workers discovered the coins inside.

They were Belgian, dated 1870, and bore the face of the then-reigning king of Belgium, Leopold II.

The gendarmerie said the workers reported the discovery and the treasure has since been put under lock and key until more information is unearthed about its provenance.

Local media reports the hoard could be worth 100,000 euros ($155,000).

Similar coins bearing the face of Leopold II have been discovered in other parts of Europe.

In 2015, amateur archaeologists discovered a hoard of gold in northern Germany dating back to the Belgian monarch's reign.

The gold coins, initially estimated to be worth more than $65,000, were found underneath a tree near Lueneburg.

The fact the coins were buried suggested they had been stolen, according to a local archaeologist.

Workmen charged with stealing treasure
The discovery in Brittany is not the first time a group of French workers have stumbled upon exactly 600 gold coins.

In 2014, three men who had been working on a house in Normandy for several days discovered a trove of gold bars and coins hidden in jars.

The six gold bars and 600 coins, dated from 1924 and 1927, were estimated to be worth more than $1.3 million.

But rather than alert the owner of the house to their find, the men stole the treasure and sold it to a coin collector.

They were later caught and charged after tax officials noticed one of the men depositing high-value cheques.