Conservative Party deputy chairman Lord Ashcroft was listed joint 65th in last year's Sunday Times "rich list", with wealth estimated at about £1.1bn.
The UK-born 62-year-old's fortune has enabled him to pay millions of pounds in donations to the Tories since the early 1980s.
He has been credited with helping to rescue the party's finances in the past, once stepping in to personally guarantee its overdraft when it was reportedly £3m in the red.
But Labour MPs have long questioned whether donations through the UK-registered company Bearwood complied with strict laws banning overseas contributions, and the Electoral Commission has now announced an investigation.
Lord Ashcroft was partly brought up in Belize and reportedly maintains extensive business interests there, and he refuses to say whether he is resident or pays taxes in the UK.
His ties with the former British colony led to a spell as its ambassador to the United Nations from 1998 to 2000.
He stepped down from the post when he was made a Conservative peer, amid considerable controversy.
The tycoon had to give assurances that he would take up permanent residence in the UK.
There was criticism of the peerage from within the Conservative Party, with former MP Sir Anthony Grant saying: "We want to detach ourselves from this notion that people only have to give money and then they can waltz into what is, after all, part of the legislature."
It is by no means the only controversy to have involved Lord Ashcroft.
His immense personal fortune has been built up through ruthless deal-making which has often shocked and unsettled the City of London.
However, even critics admit that the tycoon has a genius for making deals.
Lord Ashcroft made his first million at the age of 31 when he sold a cleaning company he had bought with a £15,000 loan five years earlier.
He then bought the Hawley Group, an ailing camping equipment manufacturer which he used as a vehicle to snap up a wide variety of companies.
In 1987 Lord Ashcroft bought the security and motor auctions group ADT for £635m. A decade later he sold it for £2.6bn, personally pocketing about £154m from the deal.
Lord Ashcroft's dealings in Belize have also generated controversy in the past, with some politicians in the central American country having suggested an influence that has been far from healthy.
The tycoon made large donations - rumoured to total about $1m - to the right-wing People's United Party (PUP) when it was in opposition.
In 1998 the PUP came to power after defeating the centre-left United Democratic Party (UDP), and subsequently introduced several pieces of legislation financially advantageous to Lord Ashcroft.
They included a law giving tax-exempt status to some companies including his offshore holding company Belize Holding Inc (BHI).
The Lord Ashcroft-owned Bank of Belize was also granted the exclusive right to set up offshore companies in Belize for US and UK citizens.
In an interview with a British newspaper around the time, Manuel Esquivel, the UDP leader and Belize's former prime minister, urged UK voters to beware.
"What happened to us should be a lesson to you," he said.
Lord Ashcroft has been Tory deputy chairman since 2005, having previously been party treasurer from 1998 to 2001.
As treasurer, responsible for fund-raising, Mr Ashcroft is said to have widened the party's pool of donors and allowed it to recruit more staff.
The now powerful party figure is credited with devising its marginal seat strategy, pouring resources into constituencies where Labour MPs are defending small majorities.
He has published several books including, in 2005, Smell the Coffee: A wake-up call for the Conservative Party.
A friend of the Thatcher family, he has said that he became a donor to the party "because of my admiration for the policies of Margaret Thatcher".
"I give the money because I support the Tory principles of free markets and personal liberty. And I do not seek anything for myself beyond the return of a Conservative government."
Lord Ashcroft settled a libel action against the Times newspaper out of court in 1999, which forms part of the subject of his book Dirty politics, Dirty times.
He founded the charities Crimestoppers and Action on Addiction, and is chairman of Ashcroft Technology Academy, in Wandsworth, south-west London.
His Who's Who entry list of recreations includes "researching the Victoria Cross", a pastime which resulted in the book Victoria Cross Heroes.
This was followed by last year's sequel, Special Forces Heroes.
He reportedly holds the largest collection of Victoria Crosses in private hands, comprising more than 150 of the medals.
Who's Who lists further recreations of "entertaining friends, trying something new, messing about in boats".