Mystery surrounds the appearance of the logo on top of the building on the isolated island of St Tudwal’s East, near Abersoch. It is owned by TV comedy writer and animal rights activist Carla Lane.
The sign was spotted by local boatmen who run trips there and upon closer inspection it appeared as though it had been nailed to the slate roof.
Roy Gregory, 66, photographed the giant ‘M’. He said: ‘We were running a trip towards the island,when I looked up and there was a McDonald’s sign.
‘Someone obviously thought it was a funny place to put a sign. But it’s a mystery how they got it there – the sea has been quite rough over the last few days.’
Mr Gregory, who operates his 30-passenger vessel Shearwater out of Pwllheli marina, believes a day-tripper must have landed on the island and put the McDonald’s sign up as a prank.
He said: ‘I have seen people on the island but in the 10 years I’ve been doing trips to the area I have never seen anyone go into the building.
‘Somebody has got that sign from somewhere. A lot of people come from Abersoch, moor their boats in Chapel Bay near this building and eat their food.
‘One of those people could have done it but we just don’t know.’
The nearest McDonalds is four miles away at Abersoch on the mainland and their sign was still in place yesterday.
The yellow ‘M’ has already attracted the interest of the local council who believe it may contravene planning laws.
Carla Lane bought the tiny island in 1992 to turn it into an animal sanctuary ‘where the wildlife could live and die as nature intended’.
A spokesman for the scriptwriter, who created the TV comedy show Bread, said: ‘She and her family are aware of it. They are not intending to talk to the media about it at this stage.’