Among their revelations, the brothers describe how they are haunted by the final phone call Princess Diana made to them from Paris, hours before her death.
The boys cut the conversation short because they were busy playing with their cousins – something that has caused them heartache to this day. 'I have to deal with that for the rest of my life,' Harry now says of the fateful decision.
He adds that: 'This is the first time the two of us have ever spoken about her as a mother. [It was] arguably probably a little bit too raw until this point. It's still raw.'
The result is the most revealing royal interview since 1995, when Diana admitted to having an affair.
William says he thinks of his mother every day and even felt her presence at his wedding. Touchingly, Harry admits that he can 'still feel the hugs she used to give us', and misses that embrace.
They paint a portrait of a devoted mother; a caring, passionate woman who had 'the ability to change the mindset of millions'.
Yet they also celebrate her impish streak, with Harry saying 'she was a total kid through-and-through' and never happier than when she was with her children driving 'through country lanes with the roof down'.
Above all, it is their painfully frank recollection of what turned out to be their last conversation with Diana that will hold millions in thrall when their interviews are broadcast on ITV tomorrow night.
The two Princes says they will never escape the deep regret they feel at hastily ending her call from Paris because they were playing in the Scottish Highlands with Peter and Zara Phillips.