Dordrecht local council has been opening up dozens of coffins at a local cemetery to monitor the decomposition of corpses without the knowledge of the relatives, an investigation has shown.

The coffins were put into underground concrete vaults rather than directly into the ground, and the conditions are not conducive to decomposition. The Essenhof cemetery has some 1,200 graves.

The situation had been known about for years, but recently questions from council members prompted an investigation. This showed the council had allowed some 30 coffins to be opened since 2013 to add different substances to speed up the decomposition process. Relatives were not told about this, local broadcaster Rijnmond reported.

Council workers also took photographs of the corpses and cemetery workers may have been exposed to health risks. Some of the graves were opened up to four times, the investigators said.

Dordrecht alderman Marc Merx said the council is “shocked” by the outcome of the investigation. “This is serious and should never have happened in this way,” he told the broadcaster.

The council had earlier tried to install a ventilation system which would have speeded up decomposition in the vaults, Merx said and the current attempts were also “well-intentioned”.

The feelings of the relatives had not been taken into account, he said, for which the council is profoundly sorry, he said.

In the Netherlands, graves are not forever. The shortage of space means graveyards are scarce – so most people tend to ‘rent’ a grave for 10 or 20 years. After that, unless relatives pay to keep up the grave, the remains will be cleared out and placed in a mass grave – and that also applies to the concrete vaults.

Dordrecht has asked for “national support” to solve the decomposition problem.