The network of offshore companies linked to the man who financed Britain’s campaign to quit the European Union has been revealed in previously unpublished documents from the Panama Papers.

The British Virgin Islands and Gibraltar emerge as key locations in the financial affairs of Arron Banks, who spent £7.5m funding Nigel Farage’s Leave.EU campaign group ahead of the Brexit referendum on 23 June. New details of Banks’s financial dealings are contained in the massive leaked database of the world’s fourth biggest offshore law firm, Mossack Fonseca, which has revealed the myriad ways in which the rich can exploit secretive offshore tax regimes.

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Banks is a close friend of the Ukip leader, revealing last month that the two men went “skinny-dipping” in Bournemouth to celebrate Farage’s short-lived retirement from heading the party. Banks has also been at Farage’s side in America, where he has been supporting Donald Trump’s presidential campaign.

The Panama Papers show that Banks is a shareholder of PRI Holdings Limited, which Panamanian-based Mossack Fonseca set up as an offshore company in 2013.

PRI Holdings is based in the British Virgin Islands (BVI), a UK offshore territory and international tax haven.

In April the BVI snubbed an anti-corruption summit held in London by the then prime minister, David Cameron. Cameron announced that foreign companies seeking to buy property in the UK would be forced to reveal their owners.

Mossack Fonseca’s leaked internal database includes around 40 entries that feature Banks. There is a colour photocopy of his passport, a gas bill from his luxury home in Bristol and a “declaration of wealth/source of funds” form. On the form, Banks says that the money entering his offshore firm comes from “business trade”.

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There is also a 2010 press cutting. It recounts how in the 1990s the former Liberal Democrat MP Mike Hancock helped a young Russian woman, Ekaterina Paderina, stay in Britain. Paderina – known as Katya – later became Banks’s wife.

Banks’s offshore arrangements are shown to be highly complex. Documents reveal that the directors for his company in the BVI are provided by a firm in Gibraltar, called Parliament Lane Nominees Limited. The firm shares an address with another firm, STM Fidecs Management Ltd, which acts as secretary to Banks’s PRI Holdings Limited. The Observer reported last year that STM Fidecs Ltd was the first to register Leave.EU as a wholly owned subsidiary. Its ownership then passed to Banks.

STM Fidecs is an offshore finance company that specialises in “international wealth protection”. It offers “international tax planning services, including providing advice on the use of trusts and companies” for its clients, which the company claims are “hi-net worth individuals”.

Facebook Twitter Pinterest Mossack Fonseca’s headquarters in Panama. Photograph: Alejandro Bolivar/EPA

Andy Wigmore, a spokesman for Leave.EU, told the Observer that Banks does not currently have a stake in STM Fidecs. However, Banks – who now wants to set up a political movement targeting Labour voters – was previously a “substantial” shareholder in the firm’s parent company.

And the Panama Papers give further details of Banks’s offshore interests. He is listed as a shareholder of PRI Holdings, together with two other people. One of them is Elizabeth Bilney, the chief executive of Leave.EU. Documents show Banks owns 25,500 shares in the firm, and Bilney 12,250.

An email sent to Mossack Fonseca includes a flowchart that sets out how Banks’s complex corporate interests fit together. Banks is at the top. An arrow points down to PRI Holdings, which is described as a “holding company for international contracts”. Other arrows go down again to four further firms: Precision Risk & Intelligence Ltd (“security and intelligence”), African Strategic Capital Ltd (“wealth management”), African Strategic Consulting Ltd (“lobbying”) and African Strategic Resources Ltd (“mining rights”). All appear to belong to Banks.

A recent profile of Banks in the New Statesman put his fortune at more than £100m. His business interests include insurance and a bank in the Isle of Man, another low-tax financial centre and UK crown dependency. He also has five diamond mines.

Wigmore said that Banks is a UK resident who pays UK tax. His latest cheque to the Inland Revenue came to £1.8m, Wigmore said, adding that PRI was an “international company” with “international clients”. It had “nothing to do with Brexit”. He added: “Arron has business interests all over the world. It’s not unusual to be mentioned in the Panama Papers. He’s an international businessman.”

Banks previously told the New Statesman: “I don’t give a monkey’s. I live in England. I pay tax in England … My position is: if you don’t like it, change the law. Business people will always go for the most efficient thing.”

There is no suggestion Banks has done anything illegal.