A "historically unique" 800-year-old stone coffin was damaged when visitors to a museum put a child inside it.
Part of the sarcophagus tumbled over and a chunk fell off at Prittlewell Priory Museum in Southend, Essex, as the Southend Echo reported.
Staff were "shocked and upset" at the "unbelievable incident", said conservator Claire Reed, who now has the job of repairing it.
Those responsible were caught on CCTV but ran off without reporting it.
The coffin was found in the grounds of the priory in 1921 complete with a skeleton which could have been a senior monk, Mrs Reed said.
Although already in three pieces, the sandstone casket has been on display since the 1920s "and nothing like this has ever happened before", she told the BBC.
The museum was very busy on 4 August when visitors lifted the child over the clear plastic barrier and into the coffin, seemingly so they could take a photograph.
"Staff heard a thump and that was the first indication something had happened," Mrs Reed said.
"It was one of those isolated, terrible incidents.
"It's a very important artefact and historically unique to us as we don't have much archaeology from the priory."
The conservator for Southend Museums Service now has to work out how to repair the damage so the coffin can go back on display "as soon as possible".
"It is repairable, and that's the good thing," Mrs Reed said.
"Public support for us has been incredible as this is so important to local people, but we will have to completely enclose it in the future.
"You can put all the risk assessments in place but you really don't expect people to try to get into the artefacts," she added.