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."
Top places for a picnic with the National Trust
Top places for a picnic with the National Trust
Sheffield Park is an expansive outdoors estate with four lakes in its centre is a great space for the kids to run around and enjoy days out this summer. Sit down for sandwiches amongst beautiful summer flowers, enjoy cricket matches on the historic pitch or explore the garden with family tracker packs and parkland trails.
There's something for all the family at Florence Court, a warm and welcoming 18th century property. The beautiful park, gardens and surrounding forest offer fantastic opportunities for all the family to enjoy. There are miles of glorious walks and cycle trails, a playground for the children and the opportunity to discover more about nature with adventure tracker packs. Take a stroll into the forest to visit the blacksmith’s forge and carpenters workshop or simply relax with a picnic in the peaceful gardens. If you want to make a weekend of it you can stay in the south wing of the house in the Butler’s Apartment. Or for country garden fans there’s Rose Cottage in the heart of the walled garden. Both cottages sleep four.
With plenty of activities to keep the children entertained and exciting walking trails to explore, Croome Park is great for a family day out and a lovely place to enjoy a picnic next to the beautiful lake. Katherine Alker, the garden and park manager, said: "My perfect spot for a picnic at Croome is sitting on the bank of the lake or in the Island Pavilion as you can listen to birds and wildlife and just relax in the tranquillity."
Whether you are cycling, exploring or strolling in this spacious park, there are plenty of opportunities to have fun with all the family followed by a relaxing picnic by the lake. The kids can also hang out on the climbing forest in the play park or pick up a family tracker pack to see what they can find. Gareth Jones, a ranger at Clumber, said: "My favourite spot for a picnic in Clumber Park is on the opposite side of the lake with views of Clumber chapel. On a warm day the sound of the country, the noise of grasshoppers in the grass and the call of swifts as they screech through the air makes it a great place to just sit and watch the world go by."
There's plenty to keep both adults and children entertained at Aberdulais Falls. Aside from the impressive waterfalls there's the tin workers' exhibition, which traces 400 years of the historic industrial site.
Nostell Priory's rolling grounds and secretive corners make it perfect for a picnic stop. There's a large park , lakeside walks and an adventure playground so the kids can let off steam as well. Mark Dudding, the parkland ranger said: "The woodlands at Nostell are an area that I like to come and relax in because it is away from the hustle and bustle of the mansion house and visitor courtyard, making it a very peaceful place to be."
Blickling Hall is one of England's great Jacobean houses, widely believed to be the home of Henry VIII’s second wife, Anne Boleyn. The beautiful parkland houses a lake and woodland and offers plenty of picnicking spots. After lunch, let the kids discover the secret garden, ancient temple and explore the citrus trees in the orangery. The whole family can also hire bikes to explore the park or play a game of croquet on the lawns. If you want to stay longer, there are seven holiday cottages on the estate, including the magnificent Tower converted from the second Earl of Buckinghamshire’s race stand.
Ancient trees, bubbling brooks and rolling lawns frame this lovely old house. Crafted from a medieval priory, Mottisfont is full of surprises like hidden doors, fun mysteries and a room where nothing is what it seems.Outside you can explore the riverside walks, the National Collection of old-fashioned roses, build dens and bring a picnic to share. Louise Govier, who works at Mottisfont said: "Underneath the Great Plane tree is the best place for a picnic. I love it because this ancient tree is always beautiful and shady in summer. The choice of view is fantastic as I can turn and look at the crystal clear River Test. Lunch spots don't get better than this."
Take the children for a fabulous day out Studland Beach, a golden sandy beach that stretches for four miles from South Haven Point to Old Harry Rocks, with shallow bathing water perfect for the little ones. Be sure to bring buckets, spades and picnics for a blissful day on the sands.
You can also visit the nearby ruins of Corfe Castle to discover over 700 years of history and relive childhood memories by seeing the inspiration behind Enid Blyton’s Kirren Castle in the Famous Five and pitch your picnic in the shadow of this impressive monument.
Enjoy a scenic picnic on the shores of Lake Windermere in the surroundings of Fell Foot Park in the Cumbrian countryside. With views of the Lakeland fells, an adventure playground for the kids and rowing boats to hire, Fell Foot is a fun place to relax with the family and it's completely free to visit.
There are six cottages on the west shore of Windermere, including a former summerhouse in the gardens of Wray Castle, a 1930s log and stone cottage and a pair of cottages just ten metres from the water’s edge. Cottages sleep between four and five so there's ample opportunity to make a weekend of your visit.