Badger Trust CEO Dominic Dyer told ITV News that Princess Anne is wrong to recommend the gassing of badgers.
"Princess Anne is fundamentally wrong," he said.
"Badger gassing does not work. It is very ineffective because you're pumping gas into a set with lots of tunnel entry and exit points.
"The gas does not disperse equally - it won't kill all the animals outright. On average 20-30% of them will die prolonged, long deaths or suffer brain damage as a consequence.
"So it's not effective and it also has huge humaneness issues associated with it as well."
Princess Anne has explained why she supports genetically modified crops - despite her brother Charles being an outspoken opponent of them.
"They do add to our ability to perhaps be more efficient users of the land," she told BBC One's Countryfile.
"I think in the long-term, when you've got the prospect of nine billion [people] to feed, you are going to need some help in doing that."
Prince Charles once warned that the development of GM crops risked creating "the biggest disaster environmentally of all time" and accused multinational corporations of conducting an experiment with nature which had gone "seriously wrong".
Princess Anne has suggested that Britain should eat more horsemeat to stop surplus animals being abandoned and said she thinks the food tastes "very good."
"An awful lot of the abandonments is because they don't perceive there to be any value in the animals," she told BBC One's Countryfile.
"The meat trade adds value to the animal so there is some point in keeping it healthy if it's got an end point that it can go to."
Asked if she had ever eaten horse meat, she replied: "Oh, certainly."
She described the meat as tasting "very good, actually."
Princess Anne has said that gassing badgers would be the most humane way to cull the animals.
The Government is considering introducing gassing after a report said that shooting badgers would not bring their numbers down enough to stop them spreading tuberculosis in cattle.
"Most of the people who did it in the past will tell you that gas is a much nicer way of doing it, if that's not a silly expression," she told BBC One's Countryfile.
"How it works is that you go to sleep, basically."
The royal owns a heard of around 30 cattle and has lost 15 rare white park cows to bovine tuberculosis in the last two years.
Gassing badgers is "the most humane way" to control their numbers, Princess Anne has said.
The Government is considering introducing gassing after a report said that shooting the animals would not bring their numbers down enough to stop them spreading tuberculosis in cattle.
"If we want to control badgers, the most humane way of doing it is to gas them," the Princess Royal told the BBC's Countryfile programme.
Defra are considering plans that could see badgers given an oral contraceptive pill to curb numbers in the ongoing debate about the badger cull.
The move is one possible alternative to culling put forward to control the spread of TB over concerns the infection passes from badgers to cattle.
Other lethal and non-lethal strategies are also under review.
Participants in yesterday's anti-badger cull protests were reportedly seen chasing BNP supporters down London's Whitehall, reports the International Business Times.
The BNP and EDL march was opposed formally by activists from groups including Unite Against Fascism and Hope Not Hate.
Rock star Brian May led around 2,000 animal welfare supporters - some wearing badger masks - in a march through central London today as they called for the Government to abort plans to cull thousands of badgers.
Thousands of people decked in black and white clothing have marched on Westminster to call for an end to plans for a badger cull.
Rock star Brian May led around 2,000 animal welfare supporters - many wearing cardboard badger masks - as they chanted "stop the cull" in protest at pilots in west Gloucestershire and west Somerset - two bovine TB hot spots, due to start from today.
The pilot culls aim to ensure free-running badgers can be killed humanely, with marksmen observed by independent experts to check they are killing the protected animal swiftly, and post-mortem examinations carried out to assess speed of death.