In lining up Sir Ronald Cohen to bankroll his leadership bid, Gordon Brown could hardly have made a more inspired choice. He is not only one of the most powerful men in Britain who combines discretion with a reputation for ruthlessness; as a former acolyte of Tony Blair, nobody else could be better calculated to put the prime minister’s nose out of joint.

The Egypt-born Jewish multi-millionaire has already given £800,000 to Labour and is said to be planning a “shadow” fundraising campaign that perfectly mirrors the one performed by Lord Levy for Blair when he was chasing power.

Lord McAlpine once dubbed himself Baroness Thatcher’s “jolly bagman”, a description that does not fit Cohen so well as Michael Levy, a pop impresario whose relentless jokes made potential donors beg for mercy.

Cohen, by contrast, employs a suave charm. “He’s delightful and very smooth, perfectly groomed and urbane,” says someone who has had professional dealings with him. “He’s also a very tough bastard. I crossed him once and he went for me.”

The 59-year-old founder of Britain’s venture capital industry might be a touch dull by Levy’s standards, which could explain his appeal to the sober chancellor, although Cohen’s film-making wife lends him a dash of Hollywood glamour.

Cohen’s rags-to-riches life has been anything but dull. Arriving in Britain as a child with only a few words of English, he became president of the Oxford Union on his ascent to the pinnacle of European finance. “There should be big rewards for men who are willing to go out and start something,” he said once. It was a maxim he put to startling effect by building his Apax business into one of the three truly global venture capital firms.

Under Cohen, Apax’s coups were legendary as it ploughed billions into 500 start-up firms, some of which became national institutions. It put money into PPL Therapeutics, which produced Dolly the sheep. Apax backed Chris Evans’s £85m buyout of Virgin Radio, later sold for a whopping £225m. Waterstone’s bookshops, Yellow Pages and the internet giant AOL have all benefited from Cohen’s Midas touch.

His efforts have brought great personal rewards. With a fortune put at £70m by The Sunday Times Rich List, he lives at an elegant £15m property in west London with his third wife, Sharon Harel-Cohen, one of the producers of the movie Gosford Park and the mother of his two children, Tamara, 17, and Jonathan, 13. Here they give lavish parties for the likes of the Rothschilds, the Rausing billionaires and Cohen’s old mentor Sir Clive Sinclair. At their home in New York they have welcomed their friends the Clintons, and their villa at Mougins is a welcoming beacon to stars at the nearby Cannes film festival.

Visitors to Apax’s London headquarters in Portland Place are disarmed by its exquisite decor and boutique-like appearance, adorned with art from Cohen’s own collection. “Everything is just so, designed to put you at ease and do a deal,” says one. “But underneath you can sense a very tough work ethic.”

In the opinion of John Studzinski, former deputy chairman of Morgan Stanley International: “He’s as tough as nails. He’s as formidable in business as he is gracious in philanthropic and social circles.”

In a rare interview, Cohen nearly admitted as much. “It’s a huge motivator to come from nothing. I’ve never really worked for anyone.”

He fell under Blair’s spell in 1996. “I decided that here was a man who was capable of implementing a programme that I have believed in for 30 years,” he declared, after sealing his conversion with a large cheque for Labour, reported to be £100,000.

This must have dismayed the Liberal party, for which he stood as parliamentary candidate in Kensington North in 1974 and would-be MEP in London West five years later.