A family damaged an 800-year-old coffin belonging to a monk after lifting a child over a museum barrier for a photograph sitting inside it.
Part of the coffin, which was already in three pieces, fell from its stand and a chunk came off it.
The people who caused the accidental damage were caught on CCTV at Prittlewell Priory Museum in Southend, Essex, but fled without reporting it.
The incident happened on August 4.
The sandstone coffin was found in the grounds of the priory in 1921 and had a skeleton inside, which could have been a monk.
Before (bottom) and after (top) a chunk of the coffin was broken off
The coffin was already in three pieces and has now been damaged further
Conservator Claire Reed, who has been tasked with repairing the coffin, said: "The care of our collections is of paramount importance to us and this isolated incident has been upsetting for the museums service, whose staff strive to protect Southend's heritage for the benefit of our visitors and enrichment of their experience within our historic sites."
In a statement Ann Holland, executive councillor for culture at Southend-on-Sea Borough Council, said the cost of repairing the coffin would be "negligible".
She said she felt the coffin needed to be completely enclosed to prevent future damage, and that the affected part of the museum would re-open "as soon as possible".
"In the meantime we would like to remind all visitors that they should observe and respect any barriers and signs in place that are there to protect our important heritage and history," she said.
Councillor Ann Holland with the broken coffin
The family did not report the damage to Prittlewell Priory
Prittlewell Priory was home to around 18 Cluniac Monks in the 13th century.
It was almost completely destroyed in 1536 and the bits that remained were drastically altered during the 18th century.
The Priory was finally rebuilt in the 20th century, but a number of original features survive.
After the dissolution of the monasteries under Henry VIII, the building was passed into private hands before the last owner, RA Jones, handed it over to the council.
The Grade I listed building is now open to the public.