It was hailed as an archeological discovery of global importance showing, among other things, the oldest representation of Christ on the cross and proof that ancient Egyptian influences had survived deep in Roman Spain.

For traditional Basques the pictures, symbols and words found scraped onto pieces of third century pottery dug up near the town of Nanclares, in northern Spain, included miraculous evidence that their unique language of Euskara was far older than ever thought. Eighteen months ago the dig's director, Eliseo Gil, claimed that some finds at the Roman town known as Veleia were on par with those at Pompeii or Rome itself. Basque nationalists bristled with pride. This archeological jewel gave them a far greater claim to a distinctive, millennial and Christian culture than they had dreamed possible.

Now a committee of experts has revealed those jewels to be fakes. "They are either a joke or a fraud," said Martín Almagro, a professor in prehistory from Madrid. "How has something like this been taken seriously for so long?" The hunt is on for an archeological fraudster who defaced fragments of third century pottery with fake graffiti.

The fraudster seems either to have buried the pieces or planted them in a laboratory where experts sifted through finds. The fakes left the first people to see them swooning.

The Calvary scene was hailed as both the nearest thing mankind had to a contemporary pictorial account of the crucifixion, and proof that Basques had been relatively early Christians.

The words in Euskera, if genuine, would have predated by 700 years the previous earliest known written form of the language. The hieroglyphics caused speculation about the existence of third century Egyptologists who might have created the inscriptions to teach children.

Now experts who have studied the pieces in depth say the fakes, some of which used modern glue, should have rung warning bells immediately. References were found to non-existent gods, 19th-century names and even to the 17th-century philosopher Descartes.

Words in Euskara used impossible spellings. The hieroglyphs included references to Queen Nefertiti which would have been almost impossible to make prior to the 19th century.

The Calvary scene, meanwhile, included the inscription "RIP". "It is a formula that can only be applied to people who are dead," Almagro told El Correo newspaper. "To say that Jesus Christ is dead would be a heresy. I haven't seen anything quite so funny in the whole history of Christianity."

Local authorities and sponsors from Basque public companies have poured hundreds of thousands of euros into excavations. Last week they closed the dig temporarily. Eliseo Gil did not return calls from the Guardian but sources said those in charge were not yet fully convinced that their finds were fake.