As enviro-maniacs go, you’ve got to hand it to the butterfly brigade. They’re refreshingly jolly. They don’t chuck paint on Chelsea tractors, chain themselves to motorway bridges or use crystals as deodorant (all the ones I met were uncharacteristically fragrant).

However, they are “touched”, says David Attenborough, who has supported the cause for years. He laughs as he tells me: “We have that wonderfully eccentric British thing of devoting excessive amounts of our time to a hobby, a passion for a fragile insect – but we don’t preach.”

Until now. After decades of gentle lepidoptery, the eccentrics are entering the fray. Not only have 20 sites just been earmarked across the country as butterfly protection zones, but work has also now begun on Butterfly World.

Don’t be fooled by the mimsy name. This is a £25m, state-of-the-art facility spread over 27 acres of prime real estate off the M25 in Hertfordshire. Set to open in 2011, it will actually look like a butterfly from the air: its head an enormous glass dome kept at rainforest temperatures for the comfort of 10,000 tropical beauties; its wings numerous gardens for different breeds. Altogether, it will be the largest butterfly house in the world and expects to attract 1m visitors a year.

Its patron, Emilia Fox, in full English-rose mode, was on hand to turn the first sod as the work began. The actress tells me: “There’s something veryusabout butterflies, isn’t there? They’re such a symbol of England – tasteful, rural. But now they’re becoming a symbol of what we’ve lost, of how all of that past beauty is ebbing away.”

Leaving aside this nostalgia for long-grassed Thomas Hardy childhoods of yore, the butterfly issue is deadly serious. As they are keenly susceptible to harmful shifts in the environment, they are considered to be the coalmine canaries of the countryside. If they die, what – or who – will be next?

“There’s no mystery about this,” says Attenborough. “The extreme pace of change has seen the ripping up of hedges, the increasingly industrialised nature of agriculture, the chemical pesticides that wipe out so-called weeds and the general expanse of concrete taking over the country – and [all that] has destroyed the breeding grounds.”

We could soon, he says darkly, be entering a “postbutterfly era”.

The stats are not encouraging. In the past 100 years, five species of butterfly (to say nothing of 60 species of moth) have been lost. Of the 54 remaining breeds, seven out of 10 are now in decline, with varieties such as the purple emperor, which lives in the tops of oak trees and flies like a jet plane, on the brink of extinction. In the past decade others have gone into freefall: 79% of high brown fritillaries and 65% of wood whites have been lost.

Now it’s down to a merry band of amateur devotees to save them. Butterfly World’s other patrons include Attenborough himself, the botanist David Bellamy, the journalist Rosie Boycott and the novelist and gardening expert Alan Titchmarsh (perhaps seeking amends for carpeting Britain in unfriendly decking and concrete water features during his Ground Force years). Principal among them is the founder Clive Farrell, a millionaire property developer from Dorset. The polite way of summing up Farrell’s affection for butterflies would be “giddy”. Others say that he’s mad.

“I’m going to show people what they can do,” he says excitedly of the new project. “I’m going to close the 27 acres and fill them with native wild flowers so you’ll see a rich and diverse meadow next to one of the largest cities in the world. There’s going to be a series of gardens and underground caves for scorpions.”

As a six-year-old, Farrell kept a caterpillar in a matchbox and just happened to be watching at the crucial moment that it emerged as a butterfly. It was a coup de foudre, he says. Unusually, the obsession weathered his transition into adulthood. As a student in London he freaked out his peers by keeping dozens of moths in their dorm.

He is already knee-deep in his new butterfly breeding programme. “My favourite butterfly is the peacock. I love the markings on the wings because they remind me of my father, who was in the RAF. Also, they can make a noise like a woman in a silk dress – a quiet rustle,” he adds dreamily.

For Fox, too, butterfly love fluttered into her life when she was a child. “A lot of my childhood was spent in Dorset, where my parents [the actors Edward Fox and Joanna David] have a house. It is incredible countryside and, thanks to the army, largely unbuilt upon. The house hasn’t changed in 20 years: it’s very informal, the garden is wild and sometimes my father leaves the grass to grow as high as your waist.

“As a little girl, we didn’t have a telly, so summer holidays were about making the most of being outside and my mum encouraging us to find out about nature. You’d find butterflies in the house and that delicate dustiness they leave.

“These days,” she adds, with a sigh, “I’m more likely to see butterflies in a Damien Hirst painting.”

Fox is just back from filming in South Africa, where she saw “scores of flutterers”. Even in Los Angeles, where she spends part of the year with her husband – the actor Jared Harris, son of Richard – she says that you see more butterflies than you do in the supposedly bucolic fields of England.

“On Sunset Boulevard, down the middle of the road, there’s a strip made of flowers. They may do it to beautify, but it’s still flowers for pollinating. Everyone has a garden and there is a lot of pressure to keep them up. Butterflies are everywhere. Gorgeous!”

Attenborough is more egalitarian. “Well, of course I’m keen on butterflies – but only as much as I’m keen on mice and ants. They’re mysterious. We still don’t know how they survive the winter. Where do they go in November? Why do they suddenly fly out from behind the curtains at a New Year’s Eve party?

“The remarkable thing, though, is that we can do something about [their current plight]. Recently the large blue has been brought back from the dead. It was extinct in Britain – but because of remarkable research which showed that the large blue caterpillar fed on a certain kind of ant that was being killed by sheep farming, it’s been saved.

You change the grazing regime, you see the benefit.” The farming community, Attenborough says, has been largely responsive to the idea of change. But another problem, believes Farrell, lies in our gardens.

“If you add up all the nation’s private gardens, the total area exceeds the entire acreage of our national parks,” he points out. And as we don’t grow flowers as much as we used to, butterflies have nowhere to breed. “Even on a back-garden scale, we can do things for insects and butterflies. And if you get it right for butterflies, you get it right for everything.”

Having sorted out her own Notting Hill garden in west London, Fox is full of tips: “They love buddleia, Michaelmas daisies and, though unpleasant, if you keep a half-bucket of nettles, they love to lay their eggs on the hairs.”

With another £11m still to raise, Butterfly World will not be cheap. Aren’t there more serious and deserving causes that would benefit from all this cash?

"Well, I haven’t gone to lobby Shell, but it doesn’t mean that this isn’t a tragedy of the modern age,” says Fox. “It may seem whimsical but it’s very important.”

She looks a little stern: “It’s amazing what you can achieve with some good old British enthusiasm.”

THE BRITISH SPECIES IN MOST DANGER

High brown fritillary (Argynnis adippe) Numbers down by 79% since 1970 – the most serious recent decline of any British butterfly. This large insect is now found in just 50 sites in the UK.

Wood white (Leptidea sinapis) Numbers down by 65%. A delicate, low-flying butterfly famed for its “head-bobbing” mating ritual.

Pearl-bordered fritillary (Boloria euphrosyne) Down by 61%. Sports silver “pearls” on the underside of its wings and can be spotted in woodland clearings and on rough hillsides as early as April.

White-letter hairstreak (Satyrium w-album) Down by 53%. A small butterfly with a white “w” on its wings. The species was badly affected by Dutch elm disease in the 1970s, which wiped out its main food.

Duke of burgundy (Hamearis lucina) Down by 52%. This elusive butterfly survives only in small, scattered colonies, including some in the Lake District and the North Yorkshire moors. Its closest living relative is to be found in the grasslands of Brazil.

Grizzled skipper (Pyrgus malvae) Down by 49%. Known for its rapid buzzing flight, it is typically found in old industrial sites such as quarries.

Dingy skipper (Erynnis tages) Down by 48%. With its sober brown and grey colouring, this small butterfly is superbly camouflaged – but the keen-eyed may spot it as far north as Inverness.

Marsh fritillary (Euphydryas aurinia) Population numbers down by 46%. Usually found in the west of Scotland, this brightly patterned butterfly can lay up to 350 eggs in a single batch.

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