For the first time, Princes William and Harry have spoken about their regrets at not having more time with their mother, Diana, Princess of Wales.

In a new documentary, "Diana, Our mother: Her Life and Legacy", William and Harry discuss their last phone call with Princess Diana before her tragic death on August 31, 1997.

"It was her speaking from Paris. I can't really necessarily remember what I said but all I do remember is probably regretting for the rest of my life how short the phone call was," Prince Harry said.

"Harry and I were in a desperate rush to say goodbye, you know 'see you later'...if I'd known now obviously what was going to happen I wouldn't have been so blase about it and everything else," William added, "But that phone call sticks in my mind, quite heavily."

September 15, 1989: The Princess of Wales (Princess Diana) follows her sons Prince Harry (right), five years old, and Prince William, seven, on Harry's first day at the Wetherby School in Notting Hill, West London. /VCG Photo

Prince William was just 15 and his brother Harry was 12 when their mother and her boyfriend Dodi Al-Fayed were killed in a car crash in a tunnel in central Paris.

The film's executive producer, Nick Kent, told Reuters, "Nobody has ever told this story from the point of view of the two people who knew her better than anyone else, and loved her the most: Her sons."

The two princes acknowledged that it had been a hard time for them to face Diana's death and how they wished she could still be around with them. They also claimed that she is for sure the best mum in the world.

September 6, 1997, from left to right: Britain's Prince Philip, Duke of Edinburgh, Prince William, Earl Spencer, Prince Harry and Britain's Prince Charles, Prince of Wales, walk outside Westminster Abbey during the funeral service for Diana, Princess of Wales. /VCG photo

Now, both continue to keep Diana’s legacy alive by continuing her humanitarian work and taking active roles in her biggest causes. 

Prince William also said that he is "constantly talking about Granny Diana" to his two kids, George and Charlotte.

“It has been hard and it will continue to be hard. There’s not a day William and I don’t wish she was around,” Harry concludes. 

October 25, 1990: The Princess of Wales leaves St. Paul's Cathedral, London, with her sons, Prince William (R) and Prince Harry. They had been attending a service to commemorate the 50th anniversary of the Blitz. /VCG photo‍

The British royals were surprisingly very open, and did not propose any limiting topics. The documentary, premiering on July 24,  coincides with Diana's 20th death anniversary.