An offshore financier who is the brother-in-law of financial services minister Andrea Leadsom has donated £816,000 to the Conservative party since she first successfully ran for parliament at the last election.

Peter de Putron, a banker who lives in Guernsey and is married to Leadsom's sister Hayley, also made a further £1m of donations to a party-backed campaign and a rightwing thinktank. Leadsom herself said that she was unaware of the donations made by a member of her own family, but a Labour MP asked whether the payments in effect amounted to a "cash for political office" arrangement.

The payments from De Putron included cheques to the Conservative party totalling £200,000 in 2010, £66,600 in 2011, £129,800 in 2012, and £204,760 in 2013, made by a company called Gloucester Research, plus further, smaller sums to her local constituency party.

De Putron also made smaller payments directly to Leadsom to hire staff, plus £300,000 to the Conservative-backed "no to voting reform" campaign, and a total of £680,000 to Open Europe, a thinktank promoting Cameron-style reforms of the EU, with which Leadsom is personally associated. Donations to British politicians from the Channel Islands are banned. But the Leadsom payments were legally permissible because they were made through UK-registered companies.

Details of the family relationships have emerged following an unprecedented leak of the identities of thousands of wealthy offshore clients who bank with a major Channel Isles private bank.

They include political donors who have paid more than £8m since 2001, and also some of the most prominent people in British life, whose identities will be revealed by the Guardian in the coming days.

A US nonprofit news organisation, the International Consortium of Investigative Journalists, has obtained records of more than 20,000 names. The Guardian has exclusively analysed the ICIJ's data, and begins to reveal those who have had dealings with a discreet Jersey branch of Kleinwort Benson, a well-known London firm which specialises in "wealth management".

In the interests of transparency, the Guardian and ICIJ will publish lists of some of the people involved in the coming days, detailing the offshore links of donors, celebrities, judges, sports stars, business tycoons and aristocrats.

The leak comes after ministers launched a scheme for Jersey account-holders to make disclosures last year. They claim the big challenge the authorities face is secret tax evasion, which is a crime. The chancellor, George Osborne, said in April: "If you're evading tax offshore, there is no safe haven and we will find you."

But the findings contradict this picture of illegality. Many Jersey loopholes used by wealthy Britons to pass on their fortunes appear from our research to have been perfectly legal. Both the Tories and to a lesser extent Labour have benefited from political donations by such individuals.

The De Putron money came via Gloucester Research, which changed its name to GR Software & Research, then to Trenchant, and now trades under yet another name, G-Research. Not only was the main company concerned controlled by Leadsom's brother-in-law, but her husband, Ben Leadsom, is a director.

After the Guardian challenged her about the relationships, Leadsom's spokesman said: "Andrea was not aware of the size of donations made by UK companies controlled by Peter de Putron to the Conservative party, and has never been involved in any way."

He added: "She has not benefited personally from these donations and does not believe they have affected her career in any way."

Leadsom had been aware, he conceded, of the other De Putron donations to her personally and to Open Europe. But, he added: "No member of her family has ever sought to promote her political career."

The Labour MP Tom Watson said on Tuesday night: "This doesn't look right. Most reasonable people will expect Andrea Leadsom to rid herself of the suspicion that she might be the beneficiary of a 'cash for office' arrangement with the Conservative party."

Separately, Ben Leadsom is recorded as receiving offshore loans. Three loans from the Channel Islands were used by the Leadsoms through their property company to acquire buy-to-let houses in 2004, plus, in 2006, the couple's own £1.3m mansion in her rural Northamptonshire constituency.

Her spokesman said Leadsom had not been misleading when she had denied to the Banbury Guardian, her local paper, in April of this year that there was any "offshore element" to their buy-to-let business. The spokesman told the Guardian: "The word 'offshore' … is taken to mean tax avoidance. There is not, and never has been, any offshore tax avoidance."

The ICIJ has previously published secret internal records of offshore companies in the British Virgin Islands. Its director in Washington DC, Gerard Ryle, said: "We make this information available not because what we found is illegal but because we think most people would think it unfair. Tax havens allow some people to play by different rules."

At a time of debate about inequalities of wealth in the UK, the Guardian's pioneering analysis reveals how the richest families dip in and out of British jurisdiction as it suits them, exploiting what academic experts call Jersey's "fictitious space".

The Treasury minister David Gauke last year called for transparency from users of the Jersey tax haven. He said: "The time has come for those with hidden offshore interests to come forward."

There is no published register of trusts or offshore holdings, and the rich appear to have often been able discreetly to avoid taxes, particularly inheritance tax, in ways too expensive for other people to use.

Children and other beneficiaries may have often had no hand in it themselves. The Earl of Guilford, for example, said : "I inherited money from a deceased relative that was held in an offshore trust ...I have ... made full disclosure of this to HMRC. You will understand that I did not establish an offshore account and was faced with the decision as to what to do with this money."

Public opinion has changed, according to Ukip leader Nigel Farage. He last year admitted a "mistake" in setting up an offshore trust designed to avoid inheritance tax. "The world has changed. Things that we thought were absolutely fair practice...30 years ago, aren't any more." He told the BBC he was now "uncomfortable" about it. Because of the lack of transparency: "I don't think you can be in public life and have a family trust."

In the loops

The Jersey files reveal how the families of rich Britons retain and increase their fortunes. These offshore manoeuvres were all legal. Many techniques have been restricted over subsequent years, as the tax authorities attempt to keep up.

Family trusts A family trust can magically transform a British fortune into a heap of tax-free wealth legally owned by somebody else, somewhere else. Expensive to run. They can be onshore, but if genuinely managed offshore, they avoid capital gains as well as inheritance tax. A newsletter from accountants KPMG says: "In the late 1980s, an offshore trust was 'the' tax planning vehicle to have for the UK's high net worth community. …the tax saving opportunities were significant.". Labour chancellor Gordon Brown tightened up in 2006 on new trusts. The present government promised in 2013 a fresh attack on "avoidance of inheritance tax using offshore trusts".

Non-doms This unique loophole makes Britain itself a tax haven. The government grants tax concessions even to people who were born in Britain, live in Britain or hold British passports. They can nevertheless claim to be "non-domiciled", or not really British, on the grounds their parents were foreign or their true home is really elsewhere. Rules have been tightened, but many "non-doms" can still keep cash and assets for years tax-free in Jersey.

Grandchildren's education A way to avoid some income tax, bringing back cash from an offshore trust to exploit the personal tax allowance permitted to each of a number of grandchildren.

Offshore mortgages Normally need to be at least £1m to be worthwhile. Multi-currency mortgages can be used for speculative switching between currencies to generate tax-free profits. Non-doms are only normally taxed on income they bring into the UK: until recently, loan interest payments on an offshore mortgage for a British house could also be made direct from offshore funds and never brought into the UK to be taxed as part of their income.

Furbs Top executives avoided inheritance tax by pouring huge company pension payments into Jersey trusts through funded unapproved retirement benefit schemes (Furbs). Ordinary pensions get income tax relief only to a total limit of £1.65m. Furbs recipients paid income tax but much larger sums could roll up tax-free and be left to their children. Their company got tax relief too. Has now been restricted.

Employee benefit trusts Until a recent clampdown, bankers' bonuses and the like were paid into Jersey trusts for the benefit of families, avoiding national insurance payments and tax. The money could then be loaned back to Britain, tax-free, and the loans sometimes later written-off.