Princes William and Harry open up about their mum Princess Diana on the 20th anniversary of her death

Prince William and Prince Harry are set to open up about their mother, Princess Diana, together for the first time in a documentary commemorating the 20th anniversary of her death.

A trailer for Diana, Our Mother: Her Life and Legacy was released in the UK overnight and shows both of the princes looking through an album of family photographs.

The documentary, which is due to air on Three in New Zealand later this month, will explore the relationship between the late princess and her two sons.

Prince Harry talks to his mother in 1995. Photo / Getty Images

"This is the first time that the two of us have ever spoken about her as a mother," Prince Harry said.

"She was one of the naughtiest parents. She was our mum. She still is our mum.

"And of course, as a son I would say this, she was the best mum in the world. She smothered us with love, that's for sure."

In the trailer the two royals can be seen looking at Diana with a young Prince William.

"Believe it or not, you and I are both in this photograph, you're in the tummy," William tells his brother.

The two have recently begun speaking about the impact of Diana's death. In April, Prince Harry revealed how William encouraged him to seek help after two years of "total chaos" as he struggled to come to terms with his mother's death.

The 32-year-old said Prince William implored his younger sibling to get help after "shutting down all his emotions" for nearly 20 years following Diana's death in 1997.

But Harry, who was just 12 years old when his mother died, admits that he ignored his grief during his teenage years, only addressing it when he was "on the verge of punching someone" as he reached adulthood.

Harry spoke of becoming "a problem" to himself, but "did not know what was wrong."

Prince Charles, Prince Harry, Earl Spencer, Prince William and the Duke of York follow the coffin of Diana in 1997. Photo / Getty Images

But 32-year-old says he was saved by going to therapy, which left him "in a good place" and wanting to help remove the stigma around mental health.

Diana was killed when the car carrying herself and her boyfriend Dodi Fayed smashed into a pillar inside a Parisian tunnel in the early hours of August 31, 1997.

The car was being driven at high speeds by the pair's chauffeur Henri Paul, who attempted to outrun the pursuing paparazzi.

Diana's death would send shockwaves around the world.

ITV's Jo Clinton-Davis said: "This new ITV film will offer viewers a fresh and revealing insight into Princess Diana through the personal and intimate reflections of her two sons and of her friends and family, many of whom have never spoken before, to bring together a definitive portrait of a unique person who touched the lives of millions."

Executive producer Nick Kent added: "This film will show Princess Diana in a way she has never been seen before, through the eyes of the two people who knew her best."