Lip curled and eyes ablaze, an image of the fiery Welsh poet and clergyman RS Thomas has inadvertently been used to advertise packets of Tyrrells sweet chilli and red pepper crisps.

Known for his bleak poetry detailing the Welsh landscape, and for his unwillingness to suffer fools gladly – he was described by chastened interviewers as "the cantankerous clergyman" and "the fiery poet-priest" – the late Welsh nationalist is an unlikely choice to adorn a packet of English crisps. But the academic Jeremy Noel-Tod noticed late last month that his image had been appropriated by Tyrrells to promote a competition. "Win a fleeting look of contempt or £25,000," runs the advertisement alongside a black-and-white photograph of a grumpy-looking Thomas.

Noel-Tod, who lectures in literature at the University of East Anglia, said he did a "double take" on spotting Thomas's picture. "My reaction was a mixture of real amusement at the absurdity of it and real anger that a respected poet should suffer such an undignified posthumous fate for the sake of selling overpriced fried potatoes," he told the Church Times. "The fact that they advertise themselves as 'Handcooked English Crisps' would certainly have been a red chilli rag to Thomas's fiercely Welsh nationalist views."

Noel-Tod then began to contact Tyrrells – repeatedly – about the use of the image on Twitter. "Hi @Tyrrells, should I interpret your silence re: RS Thomas as itself a 'look of contempt'? Or are you just thinking?" he tweeted to the crisp manufacturer. Then: "Hi @Tyrrells, awaiting yr answer! To quote the poet himself: 'Does no God hear when I pray?'" And "Hi @Tyrrells. I must say I'm beginning to have my doubts that RS Thomas was really much of a crisp man."

Others got involved, with the author Jenny Diski tweeting: "I don't see any conflict between bleak & beautiful portraits of his rural community and liking crisps. Salted anyway."

Eventually, Tyrrells responded. "We are humbled and sorry that we didn't recognise him sooner! Thanks for pointing it out," they wrote on Twitter, explaining to the BBC that the picture had been bought from an image library. "We were looking for a suitable photo for our on-pack competition and we selected the image based upon its fit," said a spokesman. "The connection with the late RS Thomas was not known at the time and had no bearing whatsoever on our decision process."

Noel-Tod said he felt Thomas would have been "deeply contemptuous of the whole business, though he is also reported to have a wickedly dry sense of humour in person, so he might privately have relished the way in which this facetious piece of marketing has backfired," he told the Church Times.

But "it does seem to me to raise a real ethical question about the casual appropriation of images of the supposedly anonymous dead for jocular commercial purposes," Noel-Tod added.