The unnamed 35-year-old snowboarder, who was with a male friend, was carried 150 yards down a mountain at Les Arcs after veering off the established runs. Although the risk of avalanches was high, she was not wearing a beacon. By the time a rescue dog found her beneath the snow 45 minutes later she was dead. “There was nothing we could do for her,” a police officer told The Times.

The woman’s death on Tuesday brought the number of skiers and snowboarders killed by avalanches in the French Alps this year to 50, making it the most deadly season since records were first kept, in 1971. Most victims were skiing off piste. Six were British.

The latest death prompted the French authorities to warn British skiers and snowboarders heading for the slopes in the coming weeks to observe avalanche warnings and not go off piste.

“Too many of these people think it’s a lark to ski off into the powder snow, and they have no idea of the danger that they are creating for themselves and for others,” a mountain rescue policeman at Bourg-St-Maurice said.

Very deep snows, unbroken cold and fierce winds have created lethal conditions, and there have been more than 90 small avalanches, mainly around the southern Alpine resorts. Most of the avalanches have been triggered by skiers and boarders, officials said.

Local experts said that the fashion for off-piste thrills had not grown noticeably this season, but skiers and boarders had too often ignored warnings of the exceptionally high avalanche danger and ventured off the prepared slopes.

In big resorts such as Tignes and Val d’Isère, up to half the visitors are estimated to duck under the safety ropes and seek fun beyond the protected pistes, with or without guides, at some stage in their holiday.

The authorities are taking a firmer line with skiers who set off avalanches. Last month a British instructor died in Les Menuires in Savoie when a group of snowboarders above him apparently set off a snow slide that buried him. The boarders are now being investigated by the police.

The worst previous French season was 1980-81, when 47 died in avalanches. Last year 22 were killed. Tignes, a resort developed in the 1970s, has suffered most this season, with three fatal accidents, but many have taken place in small resorts lacking the search and rescue resources of the big stations.

Speed is of crucial importance in saving victims, who stand little chance after more than 15 minutes buried under snow. Most die not from straight asphyxiation, as previously believed, but from breathing their own carbon dioxide, according to new research.

Almost half the fatal avalanches were at resorts in Savoie. Snowslides have swept away climbers, a ski patroller and a mountain rescuer but most victims were holiday skiers.

Jean-Lou Costerg, director of pistes for Val d’Isère, blamed this year’s severe conditions for the lethal toll and noted that the victims included several experts. Some were guides accompanying clients off piste. “Without doubt there is stronger pressure from clients who do not want to be disappointed,” he said. Major Pierre Durand, of the police Alpine rescue squad at Grenoble, said that foolhardy skiers were “encouraged by the pictures of powder snow sold by the winter sports resorts”.

The casualty rate has been less dramatic farther east. In Switzerland twenty-one people have died in avalanches this season, eight fewer than last year. Ten have been killed in Austria and sixteen in Italy.