The National Trust issued the warning ahead of European Union budget negotiations on Wednesday (29 June).
Talks would determine the future of money provided through the Common Agricultural Policy, said trust director general Fiona Reynolds.
Funding streams that had underpinned major environmental improvements in the UK countryside over the last 20 years must be retained, she said.
The Prime Minister should intervene urgently to ensure the retention of environmental schemes, said Dame Fiona.
"The narrow focus being adopted by other Member States threatens to fatally undermine these vital schemes, without adequate replacement to ensure this progress is maintained."
Without David Cameron's support, the ambitions outlined in the government's recent recent Natural Environment White Paper would remain a pipe dream.
"European funding for our countryside could suffer severe cuts, to the detriment of wildlife, rural communities and the millions who visit our beautiful landscapes each year."
Other conservation groups are also calling for the funds to be maintained.
The RSPB said scrapping payments to farmers who protected vulnerable wildlife on their land would mean an uncertain future, or even extinction for some species.
"It is imperative that we step up for nature and call on the European Commission to safeguard this vital lifeline," said an RSPB spokesman.
The Common Agricultural Policy (CAP), which determined how our farmland was managed, also had a huge influence on wildlife.
"This is not a choice between food production and wildlife - we can have both. The time is ripe to make the policy fit for wildlife, the wider countryside and for the future of farming."
Brussels should take the opportunity to ensure these reforms to the CAP reflect the needs of the environment, said the RSPB.
The European Commission said successive reforms of the Common Agricultural Policy had moved support away from production to direct income support for farmers.
This was paid provided farmers respected certain health and environmental standards, and for projects to stimulate economic activity in rural areas.
