But as he launches another takeover battle in the City - having shunned the Square Mile for more than a decade - it has emerged that Britain's 24th richest man also controls the "flag of convenience" of the central American state of Belize, which has one of the world's worst safety records.
Almost half the central American country's vessels - few of which have any real connection with the country - were detained because of safety defects between 1994 and 1996: only Honduras had a worse record.
Last year five sailors on a Belize-registered ship, the Rema, died off Whitby, North Yorkshire, when their ship sank after being detained by coastguards because it was not safe.
Mr Ashcroft's company has operational control of and a 50 per cent stake in the Belize shipping register, an official government body, responsible for maintaining safety standards in the merchant fleet.
Mr Ashcroft, aged 52, holds both British and Belize nationality, is Belize's ambassador to the UN. A tax exile who lives in Florida, he keeps most of his pounds 500m fortune offshore and is not registered to vote at the Belgravia home he maintains in this country. The six-storey house is now on the market with a price tag of almost pounds 3m.
William Hague is under pressure to sack his party's treasurer. Labour has attacked Mr Ashcroft, who has made large donations to the Conservative Party, over his control of the flag of convenience and is also urging Mr Hague to refer his funding of the party to Lord Neill's committee on standards in public life.
Born in Chichester, West Sussex, in 1946, he can stay only 90 days each year in Britain, yet is bankrolling its main opposition party with, it is said, an eye on a knighthood.
Peter Bradley, Labour MP for The Wrekin, has written to the Tory leader asking why the party is accepting money from Mr Ashcroft when the Neill committee recommended that political parties should not accept overseas donations.
Belize has yet to sign a UN Convention passed in 1986, which calls for a genuine connection between ships and their flag states and which demands that countries ensure their ships are safe.
David Cockroft, general secretary of the International Transport Workers' Federation, described the Belize register as a "sleazy" operation: "It is one of the select few registers in the shipping industry which will register anybody. It exists for no other function whatever except to make money and is used only by ship owners who have no reputation to lose."
Mr Ashcroft is publicity-shy but, in a statement, his office said reports that he gave or lent up to pounds 8m to the Conservatives were "extremely exaggerated." The shipping register was a "passive interest" in which he had no direct involvement, and the number of deaths was very low, at about one a year on average.
He has just launched a pounds 282m takeover bid for Corporate Services Group, a ploy which, in true Ashcroft style, is also complex, controversial and meeting hostility.
The City never liked Mr Ashcroft's brand of slick deals and fast fortunes in the Eighties, and it became even more wary when he relocated his business in 1984 - overnight and in complete secrecy - to the tax haven of Bermuda.
He seems to enjoy tax havens. His influence in Belize is immense. Last summer, he helped to fund the right-wing People's United Party (PUP), which comfortably saw off the main opposition, the United Democratic Party (UDP), in an election awash with cash and favours.
The champagne corks had barely stopped popping at the PUP's headquarters when Mr Ashcroft joked about the amount of cash he had pumped into the party: "If I'd have known they were going to win by so much, I'd have saved myself some money," he is reported to have remarked.
In Belize, there is a sense of deja vu. "Before the PUP came into power the last time, in 1989, Mr Ashcroft was concerned that they opposed him and he felt it important that he should have a dialogue with them," said Manuel Esquivel, the former UDP prime minister. "Since then, he is rumoured to have given them at least $1m (pounds 640,000) and now he wields enormous influence."
Mr Ashcroft was attracted by Belize's potential as an offshore tax haven - he bought its main bank, made generous political donations, drew up legislation for the government to adopt, bought 25 per cent of its telecommunications company, 20 per cent of one of its two citrus fruit producers, and was made Belize's UN ambassador.
His links with the tiny country of Belize (population 230,000) go back to his childhood when his father, Frederick, had a post with the colonial service.
Mr Ashcroft started his business career with Carreras, part of the Rothman group, as a trainee manager. After two years of boredom, he left at 26 and started a small cleaning business, which he sold to Reckitt and Colman for pounds 1m just four years later. That, however, was not enough for a man who lives for the deal.
In 1977, he bought Hawley-Goodall, a small tent-making company that diversified wildly through acquisitions, and set him on the road to his many millions, which climaxed two decades later when, in 1997, he sold his main company ADT, the security and motor auctions group, to Tyco International, a US company, for about pounds 2.5bn. Mr Ashcroft's share was pounds 154m and he still retains a half per cent interest in Tyco, worth pounds 195m.
He also owns 66 per cent of BHI Corporation (Belize Holdings Inc), the holding company for the Bank of Belize. In November 1998 his BHI stock was worth pounds 170m and his share of Carlisle Holdings, a Guernsey-based property and acquisitions group, is worth pounds 120m.
Former colleagues describe him as brilliant but ruthless. "He is cruel and he seems to get sadistic pleasure from beating the other guy to the deal," said one former executive. "He is driven not so much by the money, as the skill it takes to get it." He also has the rather odd hobby of collecting Victoria Crosses - he is said to have more than 100.
His relationship with the Conservatives has been dogged by the same kind of snobbery that drove him from the City. In spite of agreeing to underwrite up to half of the party's pounds 16m debt before the last election, he has not been universally welcomed by party members.
"You only have to look at his rollercoaster business dealings He is not an altruistic person, he is a person who pursues power. That worries many of us," said one party grandee.
Mr Ashcroft maintains an office in the Conservative Party's Smith Square headquarters and is said to have a strong influence over Mr Hague and the party chairman, Archie Norman.
Some party members and backbench MPs have privately expressed disquiet at Mr Ashcroft's central role in the party. Baron McAlpine of West Green, a former treasurer under Margaret Thatcher, blocked his appointment to the treasurer's job when he first sought it in 1990.