The Plaid office in Ammanford was painted overnight with the slogan “Cofiwch Dryweryn”.

The mural was originally painted on a wall on the road between Llanrhystud and Aberystwyth by academidc Meic Stephens in the 1960s to commemorate the drowning of the village of Capel Celyn by Llyn Tryweryn in order to supply water to Liverpool.

It has been repeatedly defaced in recent months.

Commenting on the mystery art work, Carmarthen East and Dinefwr MP Jonathan Edwards said: “ I am delighted to see this mural on the wall of my office.

"I have no idea who did it overnight but I would like to thank them”

Adam Price AM added: ”The drowning of Capel Celyn was an iconic moment in Welsh political history.

"Despite all Welsh MPs voting against the scheme, it still went ahead.

"Many people in Wales realised how powerless we were to defend our people.

"It is no exaggeration to say that this event led in the long run to the establishment of the National Assembly”

David Thomas, agent to Jonathan and Adam said: "I left the office around 5pm last night and when I returned this morning it was there.

"The only evidence we have is the initials H and E at the bottom corner of the mural”

Ammanford Mayor Councillor Deian Harries added: ”I am proud to see this mural in Ammanford.

"My thanks to the painters whoever they are”