One of Europe’s most famous nudist colonies has become the subject of an unusual investigation by French detectives after the destruction of three nightclubs in fires blamed on naturist “hardliners”.

The normally peaceful Cap d’Agde, a magnet for nudists in the south of France, has been plunged into anxiety. Investigators suspect “fundamentalist” naturists of harbouring a grudge against the échangistes, or swingers, who are drawn to the town by the promise of sex.

A so-called boîte échangiste, or wife-swapping club, called Glamour, where couples engage in group sex, was the first to be razed, in April. The next day the Palme Ré, another orgy venue, went up in flames. In September the Tantra club and Zen, its neighbouring bar, were destroyed in blazes.

Nobody was hurt but the fires have shocked a community whose crime rate has been dominated until now by cases of exhibitionism. Recently, police had to intervene as a man whipped his wife on the balcony of an apartment for the gratification of spectators in the courtyard below. “Generally we let them get on with it,” said a spokesman for the local gendarmerie. “But when there are complaints about improper behaviour, we have to act.”

Tensions have arisen between the naturists, who believe that nudity is a healthy choice of lifestyle and nothing to do with sex, and the échangistes, who are attracted to nudist camps by the prospect of multiple partners.

“What goes on in certain places is not naturism,” said Guy Delfour, former head of the French federation of naturism. “Nudity is just one element of naturism. There are other values to recognise, such as the protection of nature.”

Some in Cap d’Agde attribute the fires to fundamentalist “mullahs of chaste nudity”, as one magazine called them, who have often harangued holiday-makers venturing onto nudist beaches in bathing costumes. Others denied that naturists could have been involved.

“We don’t want to put the échangiste places out of business,” said Gilles Beaumont, a naturist and regular at Cap d’Agde. “It’s true that we don’t like being mixed up with the swingers. But we respect other people and their right to behave as they please.”