Transcript for 'The Last 100 Days of Diana'

She was called the people's princess and no member of Britain's royal family may have been more beloved than princess Diana. We all still miss her so much. The 20th anniversary of her death is this summer. Now a new documentary looks back at her final 100 days including how she pulled off a bombshell interview. This excerpt reveals how what Diana told martin Bashir convinced the queen her son's marriage was over once and for all. I was at the end of my tether. I was desperate. Reporter: She arranged to have me smuggled into kensington palace for a secret interview. Her staff were given a rare day off. If we had known about it we would have put a stop to it. The camera came up in boxes that looked like deliveries. She did her own makeup. It was planned to a tee. The prince of waste is described as a great thinker, a man with a tremendous range of interests. What did he think of your interests? Well, I don't think I was allowed to have any. Reporter: She admitted to having an affair with her riding instructor Kappes James Hewitt. Did your relationship go beyond a close friendship? Yes, it did. Yes. Reporter: Were you unfaithful? Yes, I adored him. Yes, I was in love with him. Reporter: She would never have done it, she said, if Charles had broken off with Camilla. Scandalous as her confessions were, nothing provoked more consternation at the palace than her answer to my question about the royal suck session. Do you think the prince of Wales will ever be king? I would think that the top job as I call it would bring enormous limitations to him and I don't know whether he could adapt to that. At this stage you have to remember she is still a member of the royal family. She's the mother of the future king. Reporter: For the queen, this was the last straw. A letter arrived from Buckingham palace, dear Diana, we have come to the conclusion that it is best for the country that you should divorce. This was a directive from the queen. Wow, that is riveting and fascinating to hear the lengths they went to to keep this a covert operation, that interview. Fascinating and you can tune in to the "The last 100 days of Diana" hosted by martin Bashir Sunday, 9:00 eastern and that's right here on ABC.

This transcript has been automatically generated and may not be 100% accurate.