Arron Banks’ Leave.EU campaign team met with Russian embassy officials as many as 11 times in the run-up to the EU referendum and in the two months beyond, documents seen by the Observer suggest – seven more times than Banks has admitted. The same documents suggest the Russian embassy extended a further four invitations to Brexit’s biggest funder, but it is not known if they were accepted.

It is the third time the number of such meetings has been revised upwards. For two years, Banks insisted his only contact with the Russian government consisted of one “boozy lunch” with the Russian ambassador.

After the Observer revealed a month ago that he had had multiple meetings at which he had been offered lucrative business deals, Banks told a parliamentary inquiry into fake news he had had “two or three” meetings.

Last week, when pressed by the New York Times, he admitted a fourth meeting. But the Observer has seen evidence that suggests there were at least seven more. When questioned about this, Banks offered no response.

The Observer has seen details, and the Sunday Times published an email given to it by Banks, suggesting possible deals were presented at the fourth of these meetings, on 17 November 2015, the day before the Leave.EU group – which he helped fund – officially launched its campaign.

The document suggests Banks messaged Siman Povarenkin, the oligarch introduced to him by the ambassador, saying: “I’ve chatted to Jim Mellon who is my partner in the bank and we are both interested in looking at how we could help. Jim has extensive interests in commodities.” Banks goes on to say that another business partner will be in touch “to start discussions. I’m very bullish on gold so keen to have a look”. As a postscript, he adds that “Jim knows the ambassador as well!”

Mellon is an Isle of Man-based investor who made his estimated £850m fortune in Russia in the early 1990s. The pair are close business associates, with Banks owning a major stake in Mellon’s Manx Financial Group. Mellon also donated £50,000 to the Leave.EU campaign – even though as a non-UK resident he was not eligible to vote. Mellon’s representatives say that while he recalls Banks referring to African mining interests, they were not of interest.

The New York Times reported last week that Charlemagne Capital, a fund management company founded and part-owned by Mellon, bought a multi-million pound stake in Alrosa, Russia’s largest diamond mining company, with the deal taking place just days after the referendum in June 2016.

Alrosa, a state-owned firm, is now headed by the son of one of Vladimir Putin’s closest advisers, Sergei Ivanov, who has worked with him since his days in the FSB, the Russian secret service. That investment followed an earlier one Charlemagne made in Alrosa in October 2013, when the Russian government sold 16% in the firm, more than half of which was bought by US institutions.

Representatives of Mellon said he learned of the Alrosa deal from a journalist only last week and that, although he founded Charlemagne Capital and, at the time, owned 19% of it, he was a non-executive director, had no role in investment decisions and no knowledge of the acquisition.

They said that it was normal practice for companies to offer investors new investment opportunities. Charlemagne took up one of these on 6 July 2016 with the announcement of a further privatisation of 10.9% of Alrosa in what Interfax, the state news agency, described as an “expedited” sale with shares being sold at “a knockdown price”.

The shares raised $813m for the Russian government but more than doubled in value within the space of a year. Charlemagne began selling its holding in September 2016, according to its annual report. Mellon’s representatives confirmed that he sold his stake in Charlemagne at the same time, adding that he received no personal benefit from the Alrosa deal.

Separately, documents seen by the Observer suggest that Banks and other associates, including his business partner and Leave.EU spokesman, Andy Wigmore – but not Mellon – discussed two gold deals and the Alrosa deal with Povarenkin during January 2016. This was apparently before the potential further privatisation was made public with the first reports only appearing in early February after Putin met the heads of seven state-owned firms.

The Alrosa sale was the first and biggest Russian initial public offering for many years, but others followed, including 19.5% of Rosneft, the state-owned oil firm, less than a month after Donald Trump won the US presidential election.