A ruined cottage wall painted with graffiti that has become a symbol of Welsh national pride is to be protected by a charity, its new owner has vowed, after it was repeatedly vandalised.

The stone wall, in Llanrhystud, near Aberystwyth, was first painted in the 1960s with the slogan “Cofiwch Dryweryn” (“Remember Tryweryn”), a reference to the UK parliament’s decision to allow the flooding of the Tryweryn valley in north Wales in 1965 to create a reservoir serving the city of Liverpool.

The drowning of the valley, with the loss of the village of Capel Celyn, is regarded as a turning point for Welsh nationalism, and the wall and its slogan became celebrated icons of the nationalist movement. In the decades since, the country’s most famous piece of graffiti has itself been repeatedly defaced, altered and repainted.

Following vandalism this year, however, when the wall was painted over with the word “Elvis” and later partially demolished, a local business owner has stepped in to buy the gable end, saying she will transfer its ownership to a charity in order to protect it.

The vandalism led to dozens of tributes to the wall appearing across Wales and further afield.

“I, like so many others, felt angry and hurt when the symbolic ‘Cofiwch Dryweryn’ wall was damaged twice earlier this year,” said the new owner, Dilys Davies. “It led me to think of what I could do.”

Unable to “run up to Llanrhystud late at night, climb over fences and repaint the wall”, she said, she spoke to Elin Jones, the Welsh assembly member for Ceredigion, who by chance had also been contacted by the farmers who previously owned the wall, on the main A487 between Aberaeron and Aberystwyth. They agreed to sell.

The wall and its mural are the subject of an S4C documentary on Thursday presented by the Radio 1 DJ Huw Stephens, whose father Meic Stephens, a literary editor and poet, painted the original mural, angered by the flooding of the valley.

“Although he published over 160 books, he believed that [these were] the two most important words he ever wrote,” Stephens said. His father died last year.

Gwion Hallam, the programme’s producer, said: “I think we have all been slightly surprised by the amazing reaction to the vandalism [this year]. Of course it was an act of vandalism that created the wall. I don’t think Meic Stephens thought for a second that those two words would have such an impact.”

Davies said she would hand over the wall to a charity called Tro’r Trai to secure its future, though the decision of how best to preserve it in future was yet to be made. “You could put a fence around it, but on the other hand there is something nice about the street art element and that it has been re-done after the original was done. I would like to think that, although I own the wall, it belongs to all of us.”