In 2006, during the run-up to Christmas, the Grocer magazine ran an advert for the Big Prawn Company. The ad featured a Nativity scene, but with the traditional baby Jesus replaced by an edible crustacean. The slogan read, "A King is born. Order now to ensure a Christmas delivery". Twenty-eight people complained. The Advertising Standards Authority rejected the complaints, accepting that the scenario "would be seen as light-hearted by most readers of The Grocer" and was thus "unlikely to cause serious or widespread offence."

In 2011, just before Easter, the Metro carried an ad for the mobile company Phones 4U. It featured a winking, thumbs-up Jesus and the slogan "Miraculous deals on Samsung Galaxy AndroidTM phones". Almost a hundred people complained. This time, the ASA has rejected the company's (admittedly absurd) contention that the image presented "a light-hearted, positive and contemporary image of Christianity relevant to the Easter weekend."

Instead, the regulator concludes that the adverts "gave the impression that they were mocking and belittling core Christian beliefs", "were disrespectful" and "were likely to cause serious offence, particularly to Christians".

Clearly something has changed. There were more complaints about the second ad, but given the much larger circulation of the Metro compared to the Grocer, not enough to indicate that widespread offence had been caused. Indeed, the ASA does not usually take the number of complaints it receives into account at all, even when judging whether an advertisement is likely to cause "serious and widespread offence".

Nor is it obvious why depicting Jesus as a prawn -- and the use of a non-kosher foodstuff seems especially inappropriate given Christ's Jewish background -- should be considered less offensive than a smiling, recognisably human cartoon-character offering "miraculous" deals on mobile phones. Both images are somewhat crass and likely to offend the humourless. But neither poses a serious threat to the fundamentals of the Christian faith.

It also strikes me as somewhat over-the-top of the ASA to claim that the image of Jesus emplyed in the Phones 4U ad was "mocking and belittling core Christian beliefs". The cartoon Jesus may have been based ultimately on the Roman Catholic icongraphy of the Sacred Heart. Its immediate source, however, is to be found in the 1999 film Dogma, in which a marketing-obsessed cardinal introduces the figure of a winking, thumbs-up "Buddy Christ" as an antidote to the "wholly depressing" crucifix.

"Buddy Christ" figurines and tee-shirts remain on sale, and the film, far from being banned, is shown regularly on Channel 4. The similarity between the Phones 4U advert and the Buddy Christ figure, moreover, is no accident: the one is clearly derived from the other and the cartoon would make little sense to anyone unfamiliar with the film.

It's likely that the Big Prawn complaint would have been decided differently today. In the past few years, the ASA has been taking an increasingly strict, some would say humourless, line on suggestions of religious offensiveness. It has, for example, banned a series of ice-cream adverts featuring pregnant nuns and gay priests, and even one for curling-tongs which employed the slogan, "a new religion for hair". One of the adverts deemed likely to cause "serious or widespread offence" triggered a mere six complaints. The decision led the National Secualar Society to accuse the ASA of surreptitiously re-introducing the blasphemy law.

At the very least, the ASA seems to have an alarmingly low threshold as to what constitutes "offence" where religion is concerned. An advert, it seems, need not be objectively outrageous; it's enough that someone somewhere might potentially take exception to it. The ASA's code, it is true, states that "particular care must be taken to avoid causing offence on the grounds of race, religion, gender, sexual orientation, disability or age." But it does not explain why this should be necessary, and it's hard to see why advertising should be subjected to restraints that would be considered intolerable in literature, film, art or even television.

Does it matter that the ASA is now over-protective of the supposed sensibilities of believers, the great majority of whom will at most have been mildly irritated? Perhaps not to the phone company concerned, for whom today's ruling will provide a welcome shot of free publicity. But advertising is not purely commercial. It is also public art. Its ubiquity makes it the most pervasive modern art-form, with an influence on public consciousness and the popular culture going far beyond the product being sold. The best adverts provoke thought and debate, comment on and contribute to the world we live in, and stay in people's memories long after the product being pushed has been forgotten.

Banning an advert robs people of the opportunity to have their thoughts provoked by it. Potentially it impoverishes culture. The ASA should realise that it owes greater duty to society as a whole than to the unrepresentative and eccentric handful who take the trouble to complain.