Thieves stole two artefacts dating back to the Bronze Age during a break-in at Ely Museum.
The building was broken into in the early hours of Tuesday May 7.
The items taken were the East Cambridgeshire gold torc and a gold bracelet.
Cambridgeshire Constabulary are investigating the break-in.
The museum acquired the torc in 2017 with a series of grants and donations from the public.
Elie Hughes, curator at Ely Museum, said: "We are devastated by the loss to the museum and to the local heritage of the region.
"It is a huge blow after the incredible support from the community in acquiring the torc in 2017.
"As a culturally significant object, it cannot be replaced. Our priority now is working with the police to locate the stolen objects.”
Found in East Cambridgeshire by a metal detectorist, the torc is regarded as the best to be found in England in more than a century.
It is much larger than usual examples and is made of 730g of almost pure gold.
Anyone with information should contact the police through their web chat service quoting Operation Lacunar or 101.
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