Justin Longley, the nephew of Sir Richard Dearlove — chief of MI6 at the time the coup attempt was staged last March — was a friend and associate of Mann, the Eton-educated former soldier jailed for seven years in Zimbabwe last Friday.

Longley was working closely with Mann on goldmining, forestry and engineering ventures in Africa. He visited the continent as a representative of Logo Logistics, the company through which Mann later financed the attempt in March to overthrow Teodoro Obiang Nguema, president of Equatorial Guinea.

One mining venture in Sudan also involved Sir Mark Thatcher, who was arrested last month by South African police and accused of being among the backers of the coup.

Dearlove is likely to be unhappy that controversy is dogging him even into retirement. He told the Cambridge college of which he became master last month that he was hoping for a “calmer existence” after several fraught final months as head of MI6.

A picture — the first to be published — showing him dark-suited and balding in a new edition of the Pembroke College Gazette suggests he believed he had finally escaped public attention. As “C”, Dearlove took strenuous steps throughout his career to protect his identity and gave evidence to the Hutton inquiry over a voice link.

Documents seen by The Sunday Times show Longley — the son of Dearlove’s sister — and Mann were corresponding on ventures in Sudan, the Democratic Republic of Congo, Gabon and Angola. Longley accompanied Mann on a trip to Sudan in December 2002 to inspect a mining and forestry area that could have yielded millions of pounds worth of gold and teak.

A report of the visit identified Mann as “CEO of Logo while Justin Longley is a senior project manager”. At one point Mann wrote to Longley about the fantastic potential wealth in an African gold deposit: “The grade is awesome — ounces per ton.”

Longley was connected with several business ventures that the Foreign Office was aware of or intervened in.

He worked with Mann at DiamondWorks, an Angola-based diamond mining enterprise, in the 1990s. The company had links through investors to Sandline International, the private military company that supplied arms to unseat a group of rebels who had seized power in Sierra Leone. The Foreign Office knew in advance of the operation.

Longley also worked as a manager for Oryx Natural Resources, the mining concern that had controversial links to Robert Mugabe, the president of Zimbabwe.

Oryx’s attempts to get a London stock exchange listing collapsed when the Foreign Office criticised the company’s plans for mining in the Congo — a source of “blood diamonds”. Oryx denied any wrongdoing.

Mann was jailed for attempting illegally to buy weapons for use in the coup in Equatorial Guinea. The Zimbabwean authorities claimed he confessed under interrogation that MI6 was behind his attempt to seize power in Africa’s third biggest oil producer. The Foreign Office denied it had advance knowledge. There is no suggestion that Longley had any knowledge of or involvement in the plot.

As well as Dearlove, Longley had uncles from both sides of his family in MI6. His mother was headmistress of Roedean, the girls’ public school. He also had connections with other members of the coup conspiracy. He was a close contact of Greg Wales, who allegedly helped plan the mission.

Longley is said to have regarded Wales as a “spook” and described him to an acquaintance as a man with influential connections in Washington who “worked for the CIA and the commercial wing of the Republican party”.

In February, Wales met a senior Pentagon official in the Africa department and was in touch with an analyst who worked for the CIA.

Longley left Angola in 1998 after a diamond mine where he was working was attacked by suspected Unita rebels. Eight mining personnel were killed, 24 injured and 10 went missing — presumed taken hostage during the raid.

He now works with a relative of Mann for a technical services company based in Brighton, East Sussex. Last week he declined to comment in detail on his relationship with Mann.

“I have no comment on Simon Mann and the allegations against him. My relationship with him was through DiamondWorks,” he said.

“I was general manager and he was chief operations officer. That is public record. I’ve got nothing further to say in any way, shape or form, and I would be very disappointed if any of this discussion appears in the press.”

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