Express reports the archive records hear Diana say 'the most awful' thing happened before the five-week royal tour taken by then-fiancé Prince Charles in March, 1981.
At his departure, Diana was pictured crying, and many argued her tears was brought by the idea of spending time apart from the Prince of Wales.
An official Kensington Palace official said at the time, 'We have never seen such a touching farewell.'
A woman who was at the farewell said, 'Lady Diana looked really heartbroken.'
The National Geographic has, however, changed this public opinion.
In the audio clips, Diana tells the truth behind her tears.
'You may recall seeing a picture of me sobbing in a red coat when he went off on his aeroplane. That had nothing to do with him going.
'The most awful thing had happened before he went,' the princess continued. 'I was in his study talking to him about his trip. The telephone rang, it was Camilla.
Continuing, Diana says, 'Just before he was going for five weeks. So I thought, ‘Shall I be nice or shall I just sit here?’ So I thought I’d be nice, so I left them to it.'
'And it just broke my heart, that.'