Sir Keir Starmer was urged to step in to stop a mass slaughter of ponies.
Tory leader Kemi Badenoch criticised the reported proposal for a large-scale cull of the animals on Dartmoor.
Polling experts also warned of the risks of a backlash if the cull goes ahead on the moor between Exeter and Plymouth, visited by many holidaymakers from London, other parts of the country and abroad.
Natural England is arguing that all livestock grazing on the moor should be reduced by about 75 per cent, according to The Times, to protect other habitats, plants and species.
But Mrs Badenoch posted on X: “This is total madness from another unaccountable quango.
“The government must overrule Natural England and stop it immediately.”
She added: “Keir Starmer is on his way to making his last acts in office the shameful underfunding of our military and the mass slaughter of Dartmoor ponies.”
During the 2001 general election which was hit by the foot-and-mouth outbreak on farms across Britain, the Government intervened to spare a calf called Phoenix.
Phoenix caught the affection of many people across the country after being found alive next to her dead mother five days after her herd in Devon was slaughtered.
As a row erupted over her future, Downing Street was drawn into what should happen to the animal.
On the threat to Dartmoor ponies, Luke Tryl, UK director of More in Common stressed;: “It’s a brave government that gets on the wrong side of Brits love for animals especially photogenic ones.
“Might not seem like the biggest issue but a risk this could see significant backlash from a govt that already lacks spare political capital.”
A cull could see nine out of ten Dartmoor ponies killed, according to the Times, as farmers seek to protect their own cattle and sheep to remain within Natural England’s reported limit.
Joss Hibbs, secretary of the Dartmoor Hill Pony Association, said: “Natural England is putting ponies in direct competition with commercial livestock who pay the farmers’ bills.
“Their plans disregard the scientific evidence and could decimate an endangered species that has been a feature of the landscape for over 4,500 years.”
Natural England denied it was recommending a mass cull of Dartmoor ponies but stressed that much of the moorland is currently in unfavourable condition and needed to be restored for wildlife and for the communities that depend on it.
It said: “Decisions about which animals are grazed on Dartmoor commons rest with individual landowners and commoners, not with Natural England. Our role is to provide evidence-based advice on to protect and restore designated habitats.
“We are aware of concerns that including ponies in livestock unit calculations could lead some land managers favouring more commercially profitable cattle or sheep.
“Our advisers work closely with individual agreement holders to provide tailored advice to improve nature and support businesses.”
Currently, there are less than 1,000 Dartmoor hill ponies, compared to some 7,000 in 1999, according to the Dartmoor Hill Pony Association.
