In what one archaeologist dubbed a "monumental error" the ancient tomb, that had heritage status and was therefore meant to be protected, has been completely destroyed.
Galicia's Department of Culture, Education and Universities has launched an investigation after the picnic bench - which sits on a solid concrete slab in the town of Cristovo de Cea in the northwestern region of Galicia - was placed on top of an ancient tomb, classed as "of cultural interest" by Galicia's regional government.
Local environmental group Grupo Ecolozista Outeiro reported the incident.
"The rolled concrete and modern picnic bench have caused irreparable damage, replacing what was a prehistoric cemetery of the first inhabitants of Cea..." the group wrote in a report that it sent in June to Galicia's public prosecutor, which has opened a file on the case.
The environmental group told the Spanish Huffington Post that it was proof that ancient ruins, such as tombs, petroglyphs, forts roads and mines are not being protected under the country's Spanish Historical Heritage Law.
But the mayor of Cristovo de Cea has spoken of his surprise at the investigation. José Luis Valladores claims to have had no knowledge of the ancient site before the picnic bench was constructed.
"No one told me, neither Heritage nor the environmental group," he told the Spanish Huffington Post.
"The site wasn't even marked and the logical thing would have been for them to get in contact with the local council so that we could have taken measures to protect the site," he added.
But a spokesman for Galicia's Department of Culture, Education and Universities told The Local that in 2008, the Galicia's Director General for Cultural Heritage "authorized archeological research to take place in San Cristovo de Cea", emphasizing that "the town council was well aware of its existence".
In a statement published on the town hall's website, Valladores said that the removed stones "were not part of the tomb" and the responsibility for protecting these sites rests with "the Autonomous Community", i.e the regional government of Galicia.
"I was horrified when I heard this news," Juan A Barceló, a professor of prehistory at the Universitat Autonoma of Barcelona, told The Local.
"It is a monumental error. In Spain, no-one is allowed to take the individual decision to rebuild an historical monument, specially when it is classified in the national register, as it was."
But he said that luckily such huge errors were rare, even in a country so rich in historical monuments. "Probably because it was the summer holidays, the local authority was not available and someone with no knowledge of the local heritage decided to act on their own," he told The Local.
"This is not representative of Spain where monuments over 100 years old are all preserved by law," he insisted. "All authorities, national, state and local work effectively preserving our heritage. But sometimes accidents happen. I am sure that such disasters happen in many other countries, but have not been published."