Britain's countryside faces a decade of "damaging uncertainty" unless the Government takes urgent action to support farming and wildlife in the wake of Brexit, ministers are being warned.
The director general of the National Trust said it could take up to 10 years for support packages to be put in place, so waiting to formally leave the EU will be "too late".
Helen Ghosh said affordable, high-quality food and wildlife-friendly methods can be secured if the Government maintains the £3 billion a year support for the industry and reassures farmers that food standards and environmental protections will be maintained.
Current uncertainty is prompting some farmers to revert to intensive methods for short-term profits, damaging long-term agriculture and dwindling wildlife, warned Mrs Ghosh.
"We have already seen examples of short-term decision-making, where farmers, in response to uncertainty about the future and income, have ploughed up pasture which was created with support from EU environmental money.
"It's very understandable, but heart-breaking," she said, adding that the "clock is ticking" for the Government to provide clarity before the EU cash-flow ends.
She will tell the BBC Countryfile Live event at Blenheim Palace: "We are within touching distance of a vision for the future of farming that sees thriving businesses successfully meeting the needs of the nation into the 21st century and beyond.
"The longer we wait, the more we risk losing all the gains we have made over the last decade."
A spokesman for the Department for Environment, Food and Rural Affairs said: "Leaving the EU provides us with a golden opportunity to set up new frameworks for supporting our farmers to grow more, sell more and export more great British food.
"We have committed to match the £3 billion agricultural support until 2022 and the Environment Secretary has said that support for our farmers will continue for many years to come where the environmental benefits of that spending are clear.
"As we develop this new approach to food and farming outside the EU we will not compromise on our high standards of animal welfare and environmental protection."