Offshore cash helped fund Steve Bannon's attacks on Hillary Clinton

The future White House chief strategist produced a book in May 2015 accusing Hillary Clinton of trading favours for donations to her charitable foundation. Its questionable central charge, on the sale of a uranium company to Russia, recently became the subject of a House inquiry and feverish talk on conservative media.

But the financial arrangements of another foundation, which bankrolled Bannon's creation of the book, Clinton Cash, have received less scrutiny.

Leaked documents and newly obtained public filings show how the billionaire Mercer family built a $60m war chest for conservative causes inside their family foundation by using an offshore investment vehicle to avoid US tax.

The offshore vehicle was part of a network of companies in the Atlantic tax haven of Bermuda led by Robert Mercer, the wealthy hedge-fund executive and Bannon patron whose spending helped put Trump in the White House and aided a resurgence of the Republican right.

Mercer, 71, appears as a director of eight Bermuda companies in the Paradise Papers, a trove of millions of leaked documents on offshore finance reviewed by the Guardian, the International Consortium of Investigative Journalists and other partners. The files include a copy of Mercer's US passport and other private data.

Some of the Bermuda companies appear to have been used to legally avoid a little-known US tax of up to 39% on tens of millions of dollars in investment profits amassed by the Mercer family's foundation, which funded Bannon's book and a who's who of conservative groups, along with a $475m retirement fund for the staff of Mercer's hedge fund, Renaissance Technologies.

Bill Parish, an Oregon-based investment adviser who has been consulted on the tax by US government investigators, said: "This is simple but ingenious. You take retirement plans or foundations, you invest them in a hedge fund, and even if the value rises 100%, you can sell off the investments with no tax consequences."

Mercer, who declined to comment for this article, has risen from relative obscurity to become one of the most influential figures in US conservatism. He financed ventures including the presidential campaigns of Trump and Senator Ted Cruz, the website Breitbart News and the online agitator Milo Yiannopoulos, whom he publicly disowned last week. Mercer has personally donated $41m to federal election campaigns over the past decade, according to public filings.

The company has also faced sharp criticism for trying to sidestep obligations to the public purse.