Katharine Hamnett: Katharine the great

This must be the perfect place to interview an eco-campaigner: a hot house in Kew Gardens after closing time. With most visitors departed, the residents are venturing out, and an iguana shuffles calmly across our path. "This is how it should be, isn't it?" whispers Katharine Hamnett. "Eden. A perfect ecosystem. No pesticides, just all the right predators." We wander past skeins of orchids towards some Venus flytraps. "Carnivorous plants," she mutters darkly, "remind me of the fashion industry."

Katharine Hamnett achieved stardom as a fashion designer in the early-1970s. She dressed Liz Taylor and won bundles of awards. She invented the block-print sloganeering T-shirt, then gave Mrs Thatcher a shock with one in 1984 (see overleaf). She pioneered organic cotton production in the 1990s and lost interest in high fashion as she pushed causes from the cancellation of Third World debt to electoral reform. She sent Naomi Campbell down the catwalk wearing a top saying "Use a condom" in 2004. Then Henry Holland revived her trademark huge-print T-shirts and her stock soared again.

Meeting her, you're instantly beguiled. She is one of those charmed people, tall and slim like Penelope Tree, with huge, sad, soulful eyes like Beryl Bainbridge. Out of her mouth in a very posh, low, cigarette-softened voice comes a never-ending stream of chat that combines sober fact – "over-fishing of bluefin tuna for sushi has led to a plague of jellyfish in the Med" – with delightfully batty anecdotes: "And we swam miles out to sea and got stung, and of course the best way to deal with a jellyfish sting is to pee on it, but how could we? The beach was full of paparazzi out looking for Peaches Geldof..."

She's desperately passionate – "Fair Trade? It's not good enough. It has to be organic Fair Trade. You've got 20,000 people dying every year of pesticide poisoning and you tell me" – her voice catches with emotion – "you tell me that's not a social issue?" But there's no sanctimony here. She's got a wonderfully sharp tongue. Her big-print T-shirts turned up in the Sex and the City movie – Carrie wore a Hamnett original saying "Stay Alive In 85", but is she bovvered? No. "We were probably the only designer who didn't pay to be in that dreadful film. The style's all right but the attitudes are superficial and appalling." With only a little encouragement she also slags off Carla Bruni's grey suit ("Miss Prissy! Was she trying to out-frump the Queen?") Sarah Brown ("Mrs Thundercloud!") and Cherie Blair ("How would I improve her look? Dump a bucket of mud on her head?"). Even the green and white Swiss Cheese Plant in the corner gets it: "Variegated leaves – bit naff, aren't they?"

She has that well-bred, imperious, peculiarly British type of bloody-mindedness. Dressing for our fashion shoot, she slaps on diamante brooches and bangles. "I don't want to look 'eco'. The hemp shirt thing was ghastly. It set the cause back a long way. People don't buy things out of pity. People buy the things that make their pulse race, things they love. Ethical clothes have to be beautiful and they have to be mainstream. Primark, Walmart, all the rest – they're the ones who can shift the volume and create demand. They're the ones that have to go organic." So there.

With that she notices a huge green palm marked "Titan Arum" – the famous giant lily whose flower smells of rotting flesh – and shakes her head. "No, no," she says, "That's not right," and, quick as a flash, she pulls the "Titum Arum" sign out of the peat and repositions it in another pot, the pot where she thinks it belongs. Does it matter that she isn't right about the lily? She's been right about enough over her lifetime.

Born to a socialite beauty and an RAF defence attaché in 1947, she remembers asking her mother what the London bomb craters were. Was she a bit of a crusader, as a girl? What were her causes? "Not going to school. Not going back to school." Her speciality was going over her arm with a pin "so you had this terrible rash no doctor had ever seen the like of before". Although she hated Cheltenham Ladies' College, in retrospect it served her well. "It tends to produce confident, driven women. I was driven to get out of there."

Her father's diplomatic career sent them to Bucharest, and it took her three days to get home from school on the Orient Express. "To keep myself amused I pretended to be any nationality but British," she says. "I was quite the pathological liar." Home was full of Soviet bugging devices. "Daddy would take us into the bathroom and turn the taps on if he had anything to say. How mad," she remembers, "How infantile."

Her father, ironically enough, was trading arms. "He was selling TSR-2s to the Swedes – fighter jets! We did have arguments." Hers wasn't a textbook rebellion, however: at St Martins in 1968 she found the mini-student uprising "rather irritating. I knew exactly what I wanted to be taught, I was in the right place with all the experts, fuck off and let me get on with it." The hippies, she dryly observes, "were always the richer members of the team". Her tighter budget compelled her to make her own clothes. Her parents hadn't expected her to make a career of it, though. "No one else in the family had ever been in trade, poor dears".

With a college friend, Anne Buck, she founded Tuttabankem (a half-baked anagram of their surnames) which was a rapid success, selling to Browns and Saks. They dressed Liz Taylor and Marsha Hunt. But by 1979 they had gone their separate ways and Hamnett founded her own label. Its first collection sold out at Joseph and picked up key retailers worldwide. In 1983 she started making protest T-shirts, shouting common sense in huge block letters: "Choose Life", "Worldwide Nuclear Ban Now", "Preserve the Rainforests". Today's more cynical young designers have reinvented them as decadent jokes: "Let's Play Naked Twister, Linda Evangelista". But Hamnett's were heartfelt. "Save the Seas" said one; another, even less controversially, "Save the World". '

"I don't believe my T-shirts changed anything, though they probably helped get the word out there," she says. "Hard to believe it but they used to be considered offensive. In the early-1980s American Vogue finally deigned to visit my showroom, which was at the time a very chic faux cave, and when they saw T-shirts saying 'Love' and 'Peace' they spun, literally spun on their kitten heels, and walked straight out."

Her personal environmental wake-up call had been the birth of her two sons, Samuel, in 1976, and William in 1981. "Before I had kids I was a selfish bitch from hell." Previously she had designed fur coats; now she was starting to use fashion to further her beliefs. She parted from both her husbands, graphic designer Richard Hamnett (she was born Katharine Appleton) and Jeffrey Pine, a painter. "As a mother with young children I had to work because no husband would put up with me long enough!" Looking back, she chose work. "I think men really don't like it when you're quite high-profile and making lots of money or your brain's on fire. I think it goes against their primate conditioning, and if you look at strong women they have a tough time of it. Of course, men are now seriously affected by parabens, the gender benders, so they're disadvantaged because they've ingested these hormone disrupters... I'm going to make a film for Vice magazine, which I love, asking 'Why are men such poofs?'"

In 1984 she was named Designer of the Year by the British Fashion Council, her clothes were stocked in 700 stores in 40 countries, and she went to a 10 Downing Street reception wearing one she made earlier. Earlier that afternoon, in fact. She was angry about the Falklands, the unions and missile commissioning and she was going to boycott the party. "Jasper Conran said to me, 'Why should we go and drink white wine with that murderess?' and I agreed. But then I had this idea. I went quickly to a photographic studios and had the lettering done on linen, which I had to stitch on to the T-shirt, so it wasn't far off a proper sandwich-board outfit."

In the second she slipped off her coat to reveal the protest T-shirt (pictured above right), she made fashion political, or politics fashionable. With Hamnett they're all one. Some – including Zandra Rhodes – have suggested it was bad manners to accept the prime minister's white wine then embarrass her host. Hamnett sets a certain store by manners – "You can achieve everything with please and thank you" – and this suggestion riles her. "It's not as if we were friends," she retorts. "It was a political situation. They invite you 'cos they want to look like they're friends with someone trendy and that's the price they have to pay."

She had discovered Buddhism by this time (her PR was Lynne Franks, after all) and in 1989 she commissioned a report into her company to check that her right thinking and right living was complimented by right working. "Far from managing not to harm any living thing, I discovered the human cost of cotton pesticides and through poisoning, desertification and pollution of the aquifer. I discovered workers in slave conditions." At first she thought: "I'll just tell the industry and everyone will stop." If only.

Instead she has fought to drag the industry into greener practices, taking the battle all the way to the UN (she went with Anita Roddick and loved the building – "Like being in North by Northwest") and styling her catwalk collections around her campaigns. As her official biography has it: "1990 Autumn/Winter show pushes the issue of cancelling Third World debt. Detailed information on the topic is distributed." It's hardly fashion at it's most glamorous. In the 1990s she continued to work with some of the top names in fashion, with Juergen Teller and Terry Richardson photographing her campaigns, but she was disillusioned with the industry, which she felt was conning consumers and producers.

She spent years sourcing vegetable-tanned leather, only to find one of the manufacturers had kept an "ethical" labelling but substituted it for a cheaper alternative, tanned with toxin-emitting chrome. "What's your problem?" they asked her. "The customers can't tell the difference." She walked away. In 2003, when one of her suppliers in Italy was tardy paying Senegalese farmers their organic cotton premium, she doorstepped him with a Channel 4 news crew. "Actually we doorstepped him in a limo. I leant out of the window and said, casually, 'Hi, it's only me,' and we drove in and stormed his office. It was great. Though I felt bad for the receptionist.

"I am sure," she says, breathing in the sweet, mouldy air of the Kew hothouse, "that plenty of people in the fashion industry wish I would just crawl away and die."

She has burnt a lot of bridges for her beliefs. To finance her own venture, the online eco-store Katharine E Hamnett, she had to sell her Highbury home and moved to Hackney. But hope springs eternal. She's "hot" again due to the revival of the protest T-shirts, meaning a whole new generation are discovering her; and she has faith in consumers. "Seventy-five per cent of M&S's 15 million customers have ethical and environmental concerns when purchasing," she says, the words tripping off her tongue like a mantra. "They want better conditions for workers, and they are willing to pay for them. It's profiteering manufacturers you have to work on." And she's willing to take on the very biggest suppliers. In 2006 she signed to produce a range for Tesco. The relationship suffered when she publicly accused it of green tokenism (only 44 stores carried her organic range, Choose Life) though they're "still friends". Watch this space.

In Mallorca she is creating her own green Eden. She owns a farm there – all restrained Hispano-Moorish interior, "like a 15th-century still life" – and she's trying to organise the local San Bartolomeo community into an organic "Cooperativo". "Our valley produces oranges so delicious they used to be reserved for the king of Spain. If you turn up in November they say, "We don't have any orange juice, do you mind tangerine?" Nightingales sing her to sleep; her fertiliser is parrot shit. She plans to buy some donkeys ("They'll do your work and shit for you and be your best friend") and to irrigate her land using wells with Archimedes screws powered by little windmills.

Ominous touches of Don Quixote there, but still, it sounds like paradise. To discourage house guests, she tells them she keeps a scorpion in her bathroom. "He's the size of my little fingernail, adorably brave. When I next see him I'm going to put him in a matchbox and drive to the mountains and let him free." On an enemy's land, I hope? Perhaps a profiteering manufacturer lives nearby? "Believe me, I have the best schemes and plots for people I don't like. You've seen those pressure pumps with a very fine nozzle? You get the leftover soup from shellfish and you strain it through some muslin so there's no particles and add lots of sugar, and then you put it in the pressure pump and pump it through the keyhole and it condenses on the ceiling, the walls, the chandeliers – and then the ants come..."

And Gaia shall have her revenge. Are you a white witch, I ask? "Not a white one... Of course, I don't actually do things like that. Anger is hell and you have to forgive – but you can't say I can't dream."

She's had so many causes. Does she think she would have achieved more if she's restricted herself to just one? "I don't see it as many causes, I see it holistically, as one." Has fashion made her happier, or campaigning? "It's much more exciting to get a couple of NGOs to work together, because that's very hard, than it is doing a fashion show in Paris where they all weep and tear their clothes to shreds and eat them because it's all so marvellous."

As the golden sunset flares, she starts on nuclear energy. "Did you know Gordon Brown's brother is the media head of EDF energy? Isn't that a big story? I'm ashamed to be British." Not ashamed of Kew Gardens? "No, just of Whitehall. Of Parliament Square." Would she turn down a damehood? "I'm holding out to be a Duchess. With a castle and a south-facing vegetable garden." Perhaps she should have been a politician? "Perhaps I should have been a grave-robber. A man in Turkey invited me to go grave-robbing with him once. My grandfather was friends with [Tutankhamun's discoverer] Howard Carter, and my father played with mummy beads in the sand outside Tutankhamun's tomb..."

The sun goes down on Kew but Katharine Hamnett, great English eccentric, is indefatigable. Long may she reign.