Nature-friendly farmers and conservationists in the Westcountry have called on David Cameron to strongly oppose "indefensible" cuts to rural development funds at EU budget talks this week.
In a joint letter to the Prime Minister, a group including leaders of the region's wildlife trusts have warned that pay-outs to farmers who look after the environment could be slashed by one fifth.
This would severely undermine schemes which play a pivotal role in conserving wildlife and supporting livestock production, they argue.
And such a cut would be a "body blow" to the region's most cherished places, the local wildlife and thousands of farmers who rely on the support to remain in business.
The letter urges Mr Cameron "in the strongest terms" to fight for the funding or risk damaging the "single biggest regional economic sector – tourism".
"Nature's loss will be the Westcountry's loss," it says.
Martin Harper, director of conservation at the RSPB, said conservation could "soon be much diminished".
"We wanted to know what that might mean for wildlife- friendly farmers, and I think the results of our survey are loud and clear – it would be a disaster."
The Tory leader faces calls from within his party to attempt to cut the European Union's budget at summit talks, which begin in Brussels on Thursday. This financial agreement includes spending on the Common Agricultural Policy (CAP), which is to be reformed from 2014, replacing the current Single Payment Scheme with a new Basic Payment.
Some believe farmers ought to be weaned off payments which make land viable when it would otherwise be unprofitable to work.
But in Devon and Cornwall, the payments help keep land in use in areas where neglect or abandonment could be disastrous for native species.
Upland farmers, in places like Dartmoor, and areas where wildlife rely on land being grazed could suffer.
The National Farmers' Union said it wants "market focused" reform to the CAP, to help farmers to be competitive, and called for an end to the "massively unfair advantage" enjoyed by farmers on the continent.
South West regional spokesman Ian Johnson said: "We're already at a big disadvantage, with the UK government skimming off money to fund environmental schemes and thus tilting what should be a level playing field, and we certainly don't want to see it made worse.
"If our farmers are going to have to jump an even higher environmental bar, conservation groups should be helping us get them a fairer deal."
Coastal farmer Peter French, who has helped protect rare cirl bunting which breeds in South Devon, said "the enhancement of wildlife features could become a lower priority."
Will Barnard, who manages his family's 500-acre farm in Somerset, said cuts would have "a catastrophic effect".
"We rely on this income to live, to pay our bills, and if it were cut we might have to give up farming," he added.
"We have worked incredibly hard to manage the land sympathetically and we don't want the land to disappear under intensive management."