What class of a person is that?An upper-class sort, naturally. Eco-toffs are wealthy types with impeccable environmental pedigrees. They were raised on caviar, but have developed a taste for tofu; instead of skiing at Klosters, they're more likely to be trekking across the Andes to highlight the plight of the yellow-tailed woolly monkey. They may be blue-blooded, but they're more concerned with green issues, and they are willing to use their considerable riches and high social status in the cause of saving the planet.

Bravo! Mater and pater must be very proud of their little tree-hugging lords and ladies.

The latest eco-toff to hit the headlines is billionaire David de Rothschild, heir to one of the UK’s biggest banking empires. The 30-year-old, who is regularly seen in Tatler’s list of most eligible bachelors, is about to set sail across the Pacific in a boat made out of recycled plastic bottles to highlight the problem of pollution on the high seas. He will sail through a treacherous region known as the Great Pacific Garbage Patch, a vast area of ocean northwest of Hawaii, filled with plastic waste, swirling around endlessly in a vortex caused by converging currents.

Sounds perilous. Why doesn’t he just stay at home and donate a few million to environmental causes? That would be much safer.

De Rothschild is not exactly the stay-at-home type. He’s an athlete and adventurer who became the youngest Briton to reach both poles. His boat, the Plastiki, is named in honour of the Kon-Tiki, the balsa-wood boat Thor Heyerdahl sailed across the Pacific in 62 years ago. Heyerdahl wanted to prove that South American Indians could have colonised Polynesia; De Rothschild wants to show that, if we don’t change our disposable culture and learn to recycle, we are in danger of drowning in a sea of plastic and other waste.

It’s easy for him. He can afford to go on a skite across the Pacific. The rest of us have to try to make a living and pay the bills.

Critics say that eco-toffs are using their wealth and status to play at being eco-warriors, and are preaching to the rest of us from an exalted height. They have been dubbed “ego-warriors”, chaining themselves to nuclear reactors and chasing whaling ships to satisfy their own vanity. They are Sloane rangers who fancy themselves as eco-rangers, throwing lavish fund-raising parties featuring vegan tapas and organic champagne, and getting their rich and powerful friends to donate to their pet causes (all tax-deductible, of course).

So, who are the top eco-toffs?

One high-profile green goddess is Lady Dido Berkeley, who has been campaigning for years to preserve the beauty of her beloved Thames. The grandmother doesn’t get her own feet wet, mind, but she does use her social standing to further the cause. “If you have six names on a list complaining about something, and one of them has got a title, people listen,” she reasoned. Another is Julia Stephenson, heir to the Vestey millions who has become a vegan activist, and who posed nude in Tatler to promote animal welfare. And there’s Zac Goldsmith, son of billionaire financier James Goldsmith, who, when he’s not campaigning for the Conservative Party, campaigns for environmental issues via the Ecologist, the mag founded by his uncle.

Well, at least they’re not all swanning around on horses being completely useless.

Some say eco-toffs are out of touch with the real world, so they shouldn’t go around trying to save it. They urge people to buy organic, when no one can afford it, and jet all over the world to preach about carbon emissions. And you don’t have to have a title to be tarred with the eco-toff brush. Middle-class activist Leila Deen was branded an eco-toff after she threw green custard over Peter Mandelson, in protest at a planned extra runway at Heathrow.

Try at work: “Okay, Tristan has agreed to live in the tree to stop the motorway, but he must have the Financial Times delivered to him every day.”

Try at home: “Hello, Daddy, I’ve been arrested for damaging a US warplane – can you ring Barack and tell him it’s all just a silly misunderstanding?”