Homer Simpson, the world's favourite couch potato, has achieved immortality not only on the silver screen but on a nickel coin where his head has replaced that of the Spanish monarch.

A sweetshop owner totting up the day's takings in northern Spain was astonished to find a euro coin on which the head of King Juan Carlos had been transformed into the unmistakeable visage of the cartoon slob from Springfield.

Don Juan Carlos's austerely regal Bourbon half-profile topped by a full head of curls had morphed seamlessly into the pop-eyed, big-nosed, bald-headed features of beer-glugging Homer Simpson, complete with his 5 o'clock shadow.

"The coin must have been done by a professional," Jose Martinez said when he found it in the cash register of his shop in the Asturian town of Aviles. "It's an impressive piece of work." He said he didn't usually examine all his coins, but having recently found three euro coins from Malta, he was taking more interest in them.

The anonymous defacer with an impish sense of humour had not taken his scalpel to the other side of the coin, which displays the map of Europe. But the high quality of workmanship has prompted speculation that other retouched euros might be in circulation.

There is even a suggestion that the defacement – an illegal offence – might be part of a republican conspiracy against the Spanish monarchy.

"I've been offered €20 (£16) for it," Mr Martinez said, after news of his unusual find rippled around the globe. He carries it around with him, as curious neighbours keep asking to have a look at it.

Thousands of bids for the doctored coin have come in from Simpsons fans in the Netherlands, Japan, Canada and the US. But, for now at least, the shopkeeper plans to keep the collector's item for himself.