The unnamed pensioner, who is 91, was questioned under caution after doing the crossword, which is valued at €80,000 (£67,000).

The artwork, entitled “Reading-work-piece”, was created by Arthur Köpcke of the Fluxus movement.

Beside the piece is a sign that reads: “Insert words”.

A police spokesperson said: “The lady told us she had taken the notes as an invitation to complete the crossword.”

And while they understood her error, officials at Nuremberg’s Neues Museum could not avoid making a criminal complaint.

But the woman told police there should be a sign to make it clear people were not meant to actually follow the artist’s instructions.

Eva Kraus, director of the museum, said: “We do realize that the old lady didn’t mean any harm.”

“Nevertheless, as a state museum couldn’t avoid making a criminal complaint. Also for insurance reasons we had to report the incident to the police.”

It will cost a few hundred euros to repair the work but the collector who presented the piece to the museum has reacted well.

“We will let the lady know that the collector took the damage to the work in good humour, so she doesn’t have a sleepless night,” Ms Kraus said.