Cardiff Castle has demanded that stolen stones from its historic walls are returned - after they were spotted for sale on eBay.
The seller listed the 11th century limestone stones for £300 on the online auction site, with the advert offering buyers "a fantastic opportunity to own a piece of history".
Now Cardiff Castle has demanded the historic stones are returned - and has threatened to involve the police.
Staff at the castle tweeted: "We have contacted the seller on eBay to request these items are returned, otherwise we'll be asking South Wales Police for some assistance.
"We're busy calculating the Keep's worth at £300 per brick."
The seller, who appears to be based in Frenchay near Bristol, put the stone up for auction but has yet to attract any bidders.
His description reads: "These historical pieces of the Cardiff Castle have been there ever since the castle's motte and shell was built in the late 11th century AD by Norman invaders on top of a 3rd-century Roman fort.
"As being the actual parts of the castle's motte and shell structure, this is a fantastic opportunity for someone who is thrilled to own a piece of history to claim this chance and own something unique.
"These limestone rocks where collected from inside the castle's motte and shell for the purpose of preserving it as a historical antique.
"The person willing to buy these rocks must have an interest in historical artefacts and should consider these objected as being more than just normal pieces of rocks."
Cardiff Castle is a medieval castle and Victorian Gothic revival mansion which attracts thousands of visitors every year.
The original motte and bailey castle was built in the late 11th century by Norman invaders on top of a 3rd-century Roman fort.