Last September Renu Mehta, a former model with Miss World looks and an all-encompassing self-belief, launched her charity, Fortune Forum, at a glitzy inaugural summit dinner at Old Billingsgate, London's former fish market. Five hundred "high net worth" guests, paying £1,000 a head, watched Bill Clinton give a keynote speech with Michael Douglas dancing attendance.

In one evening 36-year-old Mehta, a fashion entrepreneur, raised £1 million for four charities fighting poverty, pollution and disease in the developing world. It was some debut.

So what does it take to bring a billionaire industrialist, the merely rich and influential, a Hollywood movie star, and the 42nd president of the United States of America all together in one room to join forces in the fight to rid the world of HIV/Aids, prevent millions dying from fouled drinking water, and stop the rest of us trampling the planet to death with our carbon footprints? "Persuasion, commitment, passion, drive, real determination and a Rolodex," Renu divulges with deep-eyed intensity.

She had put the event together in just 100 days, endlessly spinning her Rolodex ­ "a Who's Who circle of friends" ­ filled with contacts acquired through business and evenings as an A-list socialite. Crucially, it was more than just an exercise in social networking with a charity auction thrown in:

this was high-powered networking for social change. It's a scene familiar in the US, where big benefit nights are a key element to the social season and there's long been a recognition that with great wealth comes a need to be philanthropic, but Renu Mehta's forum was the first of its kind in the UK.

"I want to change the face of British philanthropy," she says, "by bringing together captains of industry, entrepreneurs, heads of charitable foundations, some celebrity friends, intellectuals and politicians, and mobilise them to tackle big global issues that can't be solved by a government or a single charity." The line between business and philanthropy in Britain is blurring as never before. "I think we have reached a very significant tipping point," says Mark Astarita, director of fundraising for the British Red Cross, one of the charities to benefit from the Fortune Forum launch, who advised Mehta on both the event and setting up her initiative. "Major donors giving to make things happen is energising the imaginations of the new rich," he says.

Renu Mehta is a leading light in a growing band of young British "new philanthropists", for whom it is not enough just to write a cheque or pose with developing-world orphans to make themselves feel better. Like her, most of them are self-made, and whether they are hedge funders, bankers, corporate raiders, venture capitalists, dot-com millionaires, fashion tycoons or global magnates, they are used to seeing their cash make a difference ­ as well, of course, as enjoying the improved tax incentives that philanthropy now brings.

"I was growing increasingly dissatisfied with the lack of real purpose in my work, beyond being successful," says Mehta, who had made her pile as design director of her family's fashion business. "I wanted to dedicate my life to the global issues which I had previously failed to look straight in the eye." Friends told her she wasn't alone. Dubbed philanthropreneurs by some, and philanthrocapitalists by others, they are changing the face of British giving.

Scottish entrepreneur Sir Tom Hunter, who at 37 sold his sports retail business for £260 million and created a foundation that funds schools and poverty relief in Africa, says, "This philanthropy business has given a whole new meaning and purpose to wealth creation." The City hedge-fund managers who set up ARK (Absolute Return for Kids), and apply business principles and disciplines to fund-raising, raised an astonishing £18.4 million for disadvantaged children in an orgy of pledge-giving at its annual gala dinner in 2005. One bidder paid £300,000 for a yoga session with Sting.

"Generosity is fashionable again,'" says business guru Charles Handy in his book, The New Philanthropists. The fervour for giving is global, Handy says, but in Britain alone there are so many budding philanthropreneurs that the Queen's bankers, Coutts, has set up a whole new philanthropy department to advise and help "private clients develop a personal giving strategy".

But isn't having your own charitable foundation just the latest must-have accessory for those who have got bored with their property portfolios, islands, yachts and racing stables? "This is more than fashion or another dot-com bubble waiting to burst, it is a definite and sustainable trend," says Theresa Lloyd. The author of Why Rich People Give, the first major study into the giving patterns, experiences and expectations of the newly wealthy, Lloyd is the consultant the new philanthropists turn to when they want to know how and where to give. "Rich people giving to charity is not new," she says, "but this generation of entrepreneurs and those with eye-watering City bonuses are self-made, and unlike old money they don't want to pass it all on to their children. New philanthropists really want to be hands-on with both their cash and their skills, and to be able to measure the effectiveness of what they give."

Venture capitalist Stephen Dawson speaks for many of them when he says, "I had found giving to charity a pretty frustrating experience where, having donated, one was kept at arm's length. I wanted to be involved." So with his entrepreneurial colleagues Dawson set up Impetus Trust, the first venture philanthropy organisation in the UK. It not only amasses portfolios of philanthropic cash for charities ­ £10 million so far ­ but it also advises the recipients on how to "use investment capital principles to achieve a high social return".

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