The Swiss authorities defended granting permission for the hunting of three wolves that have killed dozens of sheep after fierce criticism from environmentalists.

Reinhard Schnidrig, head of hunting, wild animals and forest biodiversity at the Federal Environment Authority, said the Swiss rule was that a wolf could be hunted if it killed 25 farm animals within a month or 35 within a season.

"The wolf is an internationally strongly protected animal. We must find a way to guarantee this protection while minimising the damage to farm animals," he said in an interview posted on the authority's website. Wolves were wiped out in most of northwestern Europe a century ago, although small populations survived in Spain and Italy. Now protected, they have been returning to countries including Germany, Austria and Switzerland.

Mr Schnidrig said individual male wolves had moved over the border from France and Italy in the past decade and estimated there were now at least 11 wolves living in Switzerland and possibly another six. The animals reached the north side of the Alps three years ago and formed their first pack two years ago.

With quarter of a million sheep in the pastures of the Swiss Alps each summer, Mr Schnidrig said it was critical to improve flock protection so that wolves hunted wild animals instead.The Swiss canton of Lucerne said it had granted permission for a wolf to be hunted after it killed another three sheep. Hunters have until September 19 to kill the wolf, but it can only be shot in the area in which it attacked the sheep.

The Pro Natura environmental group said the granting of permits to kill three wolves this week made a mockery of the government's attempt to protect the animals.

"It legitimises the killing of almost half the proven wolves in Switzerland," it said in a statement. "Switzerland must take care or else the wolf will become extinct for a second time."