The famous "Cofiwch Dryweryn" mural which commemorates the flooding of a Welsh village has been defaced with a swastika and a "white power" symbol.
The graffiti on the mural beside the A487 at Llanrhystud near Aberystwyth is believed to have been painted today.
The mural, which was originally painted in the early 1960s, has been defaced many times over the years, but the latest incident is the second time that swastikas have been painted on it in the space of a year.
In response to the latest act of vandalism, Rhun ap Iorwerth MS said: "Reprehensible. Shameful. When will they learn that those of us who are disgusted by this will be bound together even stronger by it, in our determination to defeat their fascism?"
Nathan Goss, who runs a conservation group in the area, said: "As the heritage consultant leading on the wall consolidation project, I personally think that it's a crying shame that this truly iconic piece of Welsh heritage has been defaced once again."
The phrase "Cofiwch Dryweryn" refers to the flooding of the village of Capel Celyn to create a reservoir in 1965, but has since developed a life of its own and become a symbol of Welsh culture and identity.
The phrase can now be seen across the country in various forms, from murals to mugs, T-shirts and bumper stickers.
Some of these other signs have also been vandalised in recent times, including one painted on a slag heap in Bersham.
Dyfed Powys Police said officers are investigating the criminal damage, which was reported at 11:30am today.
A spokesperson for the force said: "Officers have attended the scene and enquiries are continuing.
"Anyone with information that could help officers with their investigation is asked to report it to Dyfed-Powys Police, either online at: http://bit.ly/DPPReportOnline, by email at: contactcentre@dyfed-powys.pnn.police.uk, or by calling 101.
"If you are deaf, hard of hearing, or speech impaired, text the non-emergency number on 07811 311 908. Quote reference: 126 of the 30th June."
