Media playback is unsupported on your device Media caption Frederick Forsyth: "It was very hard to say no"

The Day of the Jackal author Frederick Forsyth has revealed he was working for MI6 for more than 20 years.

The disclosure comes with the publication of the author's autobiography The Outsider: My Life.

Fans have long suspected that Forsyth, 77, acclaimed for his highly realistic spy novels, may have been involved with British intelligence.

He told the BBC it started when he was asked to send information from the Biafran War in Nigeria.

He said he was approached by an intelligence officer who asked him to "tell us what's going on" during the civil war, which lasted from 1967 to 1970.

The zeitgeist was different... the Cold War was very much on Frederick Forsyth

"For the last year of the Biafran War I was sending... both journalistic reports to the media and other reports to my new friend," he said.

He said MI6 wanted to know if it was true that many children were dying.

"The Foreign Office was denying that there were any dying children and they were passionate in supporting the dictatorship in Lagos, and it was, oddly enough, MI6 that had a different viewpoint," he said.

Forsyth said he saw "no harm" in confirming the truth that "children were dying like flies" in Biafra.

"It was controversial... but not about the security of our country," he told BBC News.

He added that, like many others, he was never paid for the work undertaken.

"There was a lot of volunteer assistance that was not charged for.

"The zeitgeist was different... the Cold War was very much on.

"If someone asked, 'Can you see your way clear to do us a favour?', it was very hard to say no."

Forsyth remains best known for novels such as The Day of the Jackal, The Odessa File and The Dogs of War.

'OK, Freddie!'

A former BBC and Reuters journalist, many of his fictional plots drew on his real-life experiences covering stories around the world.

Despite becoming an established author with the success of 1971's The Day of the Jackal - which earned Forsyth a three-book publishing deal and led to a hit film - he undertook missions to Rhodesia, South Africa and, at the height of the Cold War, East Germany.

As a kind of pay-off for his services, he said MI6 did approve passages from some of his later novels.

Forsyth said he was given a number to ring. He was told: "Send us the pages and we will vet them, and if they are too sensitive, we will ask you not to continue."

"But usually the response was: 'OK, Freddie!'"

Forsyth has sold some 70 million books, many of which have been adapted into films. His most recent novel was 2013's The Kill List.

He was awarded a CBE in 1997.