Just down the road from Gatwick, the neatly hedged English countryside gives way to an exuberant, utterly alien-looking landscape. Arable fields are obliterated by dense thickets of sallow. Eight metre-wide blackthorn hedges spill into flowery meadows. Wild pigs and red deer run rampant through ragwort, thistles and other weeds. The air is alive with birdsong rarely heard in Britain today – spectacular bursts of nightingale and the purring of turtle doves.

In barely a decade, rewilded nature has conquered Knepp Castle in West Sussex. Rewilding appears to be conquering conservation too. As Brexit and the savaging of agricultural subsidies loom, farming may also be engulfed by this new wild. But as rewilding blossoms, so do controversies. Scientists recently warned that wild boar illegally released into Scotland could carry the CC398 strain of the MRSA superbug that is resistant to antibiotics.

While many farmers denounce the folly of replacing food production with wilderness, some conservationists admit the term is divisive. Privately, senior National Trust staff say they can’t mention the R-word even though they recently launched ambitious plans to “restore” 25,000 hectares of Natural Trust land – creating new meadows, hedges, marshes and woodlands.

Last year the charity was attacked by broadcaster Melvyn Bragg and the author and sheep farmer James Rebanks for purchasing a sheep farm on Borrowdale without the farmhouse, intending to reduce sheep grazing in this part of the Lake District.

In Wales, one ecologist says the concept is alienating landowners and can’t be mentioned to farmers. In Northumberland, attempts by the Lynx UK Trust to release the wild cats have been mired in controversy. Even the harmless, herbivorous beaver is the subject of fierce debate: while it was recognised as a native animal in Scotland last year, beavers unofficially released into the river Otter in Devon roam free only on a government trial.

“For us it is strange to see the British struggling with the beaver. Come on, we have thousands of them!” Dutch ecologist Leo Linnartz told a rewilding conference at Knepp Castle last week. Knepp is an intensive 3,500-acre arable and dairy farm that has been dramatically rewilded by its owner, Charlie Burrell. Linnartz says that many Dutch people objected to “nature development” 30 years ago but rewilding principles are now mainstream. “We have wolves coming into the Netherlands again and we are much, much smaller than Britain. If the Dutch can do it, everybody can do it.”

In Britain, the movement ignited by writer and environmental activist George Monbiot is popularly seen to seek the return of extinct carnivores – bears, wolves and lynx. In practice, this large-scale restoration of wildlife is returning more modest herbivores to the countryside. Since 2001 at Knepp, Burrell has set free wild herbivores – konik ponies, longhorn cattle, red deer and Tamworth pigs – to mimic the grazing of extinct aurochs, wild boar and wild horses.

Deer on the Knepp Castle estate, West Sussex. Photograph: Knepp Safaris

Nature has bounced back astonishingly quickly. Barren monocultures of maize have been replaced with a landscape that looks more like African savannah. The Knepp Castle estate boasts more purple emperor butterflies than anywhere in Britain. It’s thought to be the only place in the country where Britain’s fastest-declining bird, the turtle dove, is increasing in number, with at least 16 pairs this summer. It’s a hotspot for endangered nightingales. The air is thick with birds of prey – elusive long-eared owls, hobbies and one of the country’s only tree-nesting peregrine falcons.

A few rewilding purists scoff at reintroducing herbivorous free-roaming livestock but ecologists say that rewilding’s key principle is to replace conservation “mollycoddling” with “restoring natural processes” – allowing a landscape and its plants and animals to run wild. For decades, ecologists believed the end result would be dense forest – and a mass extinction of sun-loving wild flowers and butterflies – but this dogma has been demolished by Dutch ecologist Frans Vera. Since the 1980s Vera has introduced wild cattle, horses and red deer to 4,000 hectares of rewilded marshland at Oostvaardersplassen nature reserve, and proven that “natural” grazing creates a more dynamic landscape, a constantly changing mosaic of open glades and wooded groves.

In the Highlands, rewilding is taking a different form as large landowners such as the Danish owner of Glenfeshie estate and Paul Lister at Alladale restore ancient Caledonian pine forest by culling the historically unprecedentedly high numbers of red deer. But David Balharry, former Scotland director of Rewilding Britain, cautions that rewilding in Scotland will only be championed by policymakers and politicians when it is led by local communities.

For Burrell at Knepp, rewilding has been a pragmatic way to revive the struggling family farm. Ecotourism makes as much profit as his conventional farming business once did. And with Brexit looming, Burrell’s diversified business is no longer dependent on the disappearing EU subsidy regime.

Knepp’s unproductive heavy clay meant Burrell could not compete with globalised food production. The government’s inevitable post-Brexit slashing of farm subsidies could make hundreds of similarly “marginal” farms unprofitable. Burrell’s profits may be steady while conventional dairy and cattle farm incomes fall off a cliff but no farmers have yet followed his example.

“It takes a new eye to look at this and say, ‘that’s beautiful’, rather than go, ‘that’s just a real mess’,” says Burrell. “Other farmers may have a moral attitude towards it too – why are you stopping food production?” Burrell produces 75 tonnes of organic free-range meat each year but many farmers criticise rewilding for abandoning productive farmland when the world’s population is growing.

The purple emperor thrives on rewilded farmland. Photograph: Alamy Stock Photo

Wouter Helmer, rewilding director of Rewilding Europe, sees no conflict between food production and rewilding: Europe is heading for a future of food produced more intensively in fewer areas, releasing less productive arable land for rewilding, he says. “Farming is being done by fewer and fewer farmers on a larger scale on the best soils. They leave the less profitable lands – the Scottish Highlands or continental marshlands and river deltas become adventure land for an increasingly urban population.”

Helmer says there is no point in seeking to feed the world with traditional organic farming because there is no one to do the labour: when he asks Dutch students who wants to farm, none raise their hands. “They have a completely different relationship to nature to their parents or grandparents. They are not fighting with it on a daily basis. On one hand they are disconnected from nature but on the other hand they are becoming more relaxed with nature – it’s hunting and gathering but hunting with a camera and gathering experiences. Corporate training or meetings are held outside. The part of the countryside which is not used for intensive farming starts to serve all these new urban needs.”

At the Knepp conference, ecology professors reported that students were newly inspired by rewilding but some environmentalists worry how it connects with urban populations. “The challenge is how to make rewilding an issue that people in their ordinary lives can take action on,” says Elaine Gilligan of Friends of the Earth. “It has a real wow factor for engaging people in nature but how can rewilding be taken to the streets of Birmingham?”

Rewilders argue that reducing flood risks for cities is one practical way rewilding can enhance urban life. The Yorkshire Peat Partnership is restoring 30,000 hectares of upland bogs which act as giant sponges for floodwater. Conservationists are also working with Leeds city council on proposals to rewild the upper river Aire – adding meanders, planting trees and creating new washlands – to reduce the floodwater flows through Leeds.

As well as reducing flood risk, rewilding alleviates soil erosion, says Ted Green, founder president of the Ancient Tree Forum. Intensive farming can worsen flash flooding, and cause fertile soils to be swept out to sea. “The land may belong to the landowner but the soil must belong to the nation,” says Green. “We are fertilising the ocean. When you see people cleaning out their houses after floods, you don’t see them removing water, you see them removing mud. If we used the word ‘mud’ instead of floodwater, it’s no longer an engineering problem – it’s a farming problem.”

As conventional conservation jumps on the rewilding bandwagon, Miles King, founder of the charity People Need Nature, fears it could become a cliche. A project to release water voles into Northumberland was recently described as rewilding. “That’s just a basic conservation project,” says King. “The risk is you end up rebadging all these conventional nature conservation activities as rewilding in the hope that people take an interest. We need to avoid being sucked into that.”

And some conservationists worry that rewilding could replace the traditional protection of rare species on small nature reserves. “If rewilding really takes off there’s a risk people will say, ‘Oh we don’t have to do any of that old stuff,’” says Matt Shardlow, chief executive of Buglife. “But we still have habitat fragmentation and species in tiny places and we have to take care of them even if you have some areas made bigger for wildlife.

“If in 100 years time you have a Knepp in every parish, maybe some species can come off the endangered lists and won’t need such precious care.”

As turtle doves purred in the burgeoning thickets at Knepp, the conservationist Sir John Lawton, the author of the influential government report, Making Space for Nature, led a rallying cry for “more, bigger, better and joined” wild areas. “We need more Knepps and Ennerdales. If we can do impressive mid-range rewilding somewhere as close to Gatwick airport as this, we can do it anywhere. This is the south-east of England for crying out loud! Let’s not pretend we can’t make space for nature because we can and we must.”

HOW YOU CAN HELP

Purists wouldn’t call it rewilding but some councils are relaxing their management of verges and parks, letting grass grow longer and allowing wild flower areas to develop. Burnley borough council estimates it is saving £58,000 a year by reducing grass-cutting to benefit wildlife. Councils including Dorset, Devon, Cornwall, East Sussex and Bristol have adopted “pollinator action plans” with strategies for wilder areas to encourage bees and butterflies. Friends of the Earth is campaigning for more local action for on pollinators.

Gardeners are unlikely to be able to reintroduce wild boar (rewilders say you need a minimum of 120 hectares to create a large enough area for wild grazing) but we can at least liberate our lawns. Instead of unleashing wild ponies, sow yellow rattle, a flower that is a parasite of grass and creates space for wild flowers and herbs. Long grass is the food plant for butterfly caterpillars including meadow browns, ringlets and marbled whites, and home for grasshoppers and meadow ants that attract green woodpeckers.

Many rewilded Dutch reserves are coastal and riverine areas close to cities and towns and some of the most exciting rewilding in Britain is happening on the Thames estuary. The RSPB has returned the arable flatlands of Wallasea Island to salt marsh, while rewilded brownfield nature reserves such as Canvey Wick are “brownfield tropical rainforests”, rich in rare invertebrate species. Even London has new wild habitats, such as Woodberry Wetlands in north London, where bittern lurk in new reedbeds.

There are dozens of exciting habitat creation or restoration projects under way across Britain. The Great Fen Project in Cambridgeshire seeks to connect the last fragments of undrained fen and restore a unique wetland once home to the extinct large copper butterfly. The great bustard has been reintroduced to Salisbury Plain. The coastline around Medmerry, East Sussex, has been realigned to expand what is the last great wilderness in southern Britain – salt marsh. There are new woodlands, such as Carrifan Wildwood in Scotland’s southern uplands and rewilded estates such as Glenfeshie and Knepp can be explored via the public foopaths that cross them.

You can join rewilding campaigns by volunteering for Rewilding Britain, the new charity inspired by George Monbiot.