'I'd cycle in, the recorder was in the briefcase': How the explosive Princess Diana tapes were secretly made without the royal family's knowledge

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This is the astonishing 'story behind the story' of how Princess Diana's tapes were secretly recorded and shared with the world.

Speaking with Liz Hayes on 60 Minutes on Sunday, journalist Andrew Morton revealed details of the covert operation he undertook to gain access to Princess Diana without the royal family's knowledge.

Morton said he befriended James Colthurst, a doctor and long-time friend of Princess Diana's, who agreed to be an undercover agent for him.

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The story of how Princess Diana's story came to be told in a series of tapes is finally revealed

Colthurst told 60 Minutes he rode his pushbike to the palace under the pretense of meeting Diana for a friendly lunch – while secretly making recordings. 

Colthurst said Princess Diana was extremely passionate about telling her side of the story, despite its sensitive nature.

'She was enormously enthusiastic to have her story out there, she knew exactly what she was doing,' he said.

Armed with a list of questions from Morton, Colthurst recorded the princess's answers.

'I'd cycle in, the recorder was in the briefcase, nothing surprising there. 

'I'd go in and we'd normally have a few questions before lunch, we'd have lunch then we'd come out after lunch, I'd clip the microphone on and she'd finish them off,' he said.

'I'd cycle in, the recorder was in the briefcase, nothing surprising there,' long-time friend James Colthurst (pictured) said 

Mr Colthurst said 'there were always people there curious to know what was going on' when he visited Princess Diana to record her 

'There were always people there curious to know what was going on… (I was asked) "was there a particular reason for lunch today?" and I said "yep I'm hungry".' 

Colthurst also said he and Princess Diana used 'scramblers' to record their phone conversations. 

'I had one and Diana had the other one, she'd put it over the phone like this. That's encrypted between the two. It's really primitive technology for now days, but at the time ... it worked very well,' he said.  

Colthurst told Liz Hayes he and Princess Diana used 'scramblers' to record their phone conversations

Morton, who wrote the tell-all book in 1992, said its making was a 'ramshackle procedure'.

'(Colthurst) would put on a microphone then he'd munch his way through a plate of biscuits then just read out the questions,' he said.

Morton recounted the moment he listened to the first tape and realised the gravity of Diana's story.

'I'll never forget listening to the first tape which was in a working man's café,' he said.

'All around everyone's eating bacon and eggs chatting away, and I put these headphones on and turn on the tape recorder and listen to Diana talking about bulimia nervosa, which I'd never heard of, talking about her suicide attempts, talking about this woman called Camilla Parker Bowles.

'It was like entering a parallel universe, I walked out of that café thinking, wow what on earth have I heard.'

Princess Diana pictured with her then fiance Prince Charles at Buckingham Palace

'It was the most incredible outburst of really inner most pain rage frustration anger, you were swept away with it. It was very compelling,' Morton said 

Morton said Princess Diana spoke with a 'breathless urgency'.

'Almost like someone who was a prisoner who had a few moments before the guards came,' he said.

Morton said there was no doubt in his mind Diana was speaking the truth.

'It was the most incredible outburst of really inner most pain rage frustration anger, you were swept away with it. It was very compelling,' he said.

Morton said there was no doubt in his mind Diana (pictured in 1983) was speaking the truth

Prince Charles and Princess Diana pose by the River Dee while on their honeymoon

Both colthurst and Morton said they knew how shocking the tapes were and understood the seriousness of what they were doing.

'From the very moment I heard that first tape I was looking over my shoulder and being very watchful and very careful,' Morton said.

Five years after the book was published, Princess Diana was killed in a horrific car crash in Paris.

Five years after the book was published, Princess Diana was killed in a horrific car crash in Paris

Now 20 years after her death, the tapes have been rebirthed, and Diana's story retold in a new book released by Morton.

Morton said it was the nearest thing to Princess Diana's biography.

Despite the negative attention he received for sharing Diana's story with the world, Morton said he had no regrets.

'I've often wondered, would I have done it again? I think honestly the consequences of not doing it were going to be too tough I think,' he said.