Day of the Jackal author reveals in autobiography that he worked for the intelligence service for more than two decades

Frederick Forsyth will admit in his forthcoming autobiography that he worked as an agent for MI6 for more than 20 years.

The bestselling thriller author, who was an RAF pilot and a journalist before turning to fiction with The Day of the Jackal, is due to release The Outsider next week. Forsyth has previously denied claims that he worked for MI6 – “Some said that I was a spook, but I just knew a few,” he told the Guardian in 2001 – but an extract from his memoir in the Sunday Times reveals how in late 1968 a “member of the Firm” - MI6 – called Ronnie sought him out.

The Nigeria-Biafra conflict had been ongoing for 15 months, and Ronnie needed “an asset deep inside the Biafran enclave, what he termed ‘someone in on the ground’”. Forsyth had been reporting from Biafra as a freelancer, and writes that “when I left for the return to the rainforest, he had one”.

Forsyth says that he was simultaneously working as a stringer for various newspapers and magazines reporting on the conflict and the humanitarian disaster, and keeping Ronnie “informed of things that could not, for various reasons, emerge in the media”.

He told the BBC that he was not paid for the work he did. “There was a lot of volunteer assistance that was not charged for. The zeitgeist was different … the cold war was very much on,” he said. “If someone asked: ‘Can you see your way clear to do us a favour?’, it was very hard to say no.”

Forsyth’s reporting from Biafra provided the material for his first book, The Biafra Story, a non-fiction account of the breakaway state’s war with Nigeria. He also undertook fact-finding missions to Rhodesia and South Africa, and in 1973, two years after the publication of his debut novel The Day of the Jackal, went into East Germany to retrieve a package from an asset. He played the part of a British tourist visiting the Albertinum museum. “Graeco-Roman treasures were my new enthusiasm and there were books to study as if for an exam,” the author told the Sunday Times, which said that Forsyth was handed the files under a toilet door at the museum.

Forsyth said this weekend that he was making the revelation now because “it is 55, 60 years later. There have been memoirs written, highly secret minutes have been published. There’s no East Germany, no Stasi, no KGB, no Soviet Union, so where’s the harm?”

He also revealed that he had consulted MI6 over passages in his novels, which are known for their authenticity. “I had a number to ring,” he told the Sunday Times. “I would have a lunch at the club, I’d ask is it OK? They would check with their superiors, and then they would say yes, you can use that, with one proviso, that sheets must be provided for vetting – just in case I went too far.” Usually, he told the BBC, “the response was: ‘OK, Freddie!’”

The 77-year-old Forsyth is the author of 13 bestselling novels, including The Odessa File, The Dogs of War, The Devil’s Alternative and The Fourth Protocol. The Outsider will be published on 10 September, described by its publisher as “a candid look at an extraordinary life lived to the full, a life whose unique experiences have provided rich inspiration for 13 internationally bestselling thrillers”.