The Exmoor Emperor, a giant red stag which is thought to have been the biggest animal in Britain, has been found shot dead in the West Country.

The creature, which weighed more than 135kg (300lb) and stood nearly 2.75 metres (9ft) tall, was killed close to the busy Tiverton to Barnstaple road in the middle of the annual rut.

Its death – thought to be down to a licensed hunter rather than a poacher – has infuriated deer experts and wildlife enthusiasts who say wild red stags should be protected during the mating season.

Red deer stags are the biggest indigenous land animal left in Britain. The Exmoor Emperor, who was given his nickname by photographer Richard Austin, was believed to be the largest wild animal in the country today.

Peter Donnelly, an Exmoor-based deer management expert, said it was a disgrace the animal had been shot during the mating season.

"It could be that he didn't get a chance to rut properly this year, therefore his genes have not been passed on this time round," he said.

"The poor things should be left alone during the rut, not harried from pillar to post. If we care about deer we should maintain a standard and stop all persecution during this important time of the year."

A deer enthusiast who did not want to be named said a group of people were out watching stags earlier this month close to where the Emperor was killed. The man said a shot had been heard very close to the Tiverton to Barnstaple road.

He said that although the shooting was legal, deer lovers were becoming increasingly worried about the number of rich sportspeople coming to Exmoor to shoot its finest stags as trophies.

Donnelly, from Dulverton in Somerset, agreed that competition for stags was becoming intense. "There are people who are prepared to spend quite ridiculous sums of money to have a trophy on their wall. People talk about £1,000 for a good head, but I've heard there are those who will pay a lot more," he said.

He said some older stags needed to be culled after a certain age, but the Emperor had been in good health. "Yes the Emperor was starting to get past his best, but he was definitely not at that stage yet," he said.

Still, Donnelly added, the empire may yet survive: he said he had seen a very large young stag in the last few days – a magnificent beast that could well be the result of the Emperor's activities during a previous rutting season.