An 800-year-old coffin was damaged when a family lifted a child over a museum barrier and into it for a photograph.
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 earlier this month.
The sandstone coffin was found in the grounds of the priory in 1921 and had a skeleton inside, which could have been a monk.
Conservator Claire Reed, who has been tasked with repairing the coffin, said:
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.