The former Lady Diana Spencer, whose fairytale marriage to Prince Charles captivated the world until it publicly unravelled with infidelity and divorce, died in a car crash on August 31, 1997.
Five years ago, her sons attended a series of public commemorations of her life, and spoke of the enduring effect of her death on them.
This year the brothers will not take part in any official commemorations, preferring instead to mark the occasion in private with their own families.
William, 40, recently announced a move from London to the Windsor estate west of London with his wife, Catherine, and their three young children.
Harry, 37, now lives in California with his wife, Meghan, and their two children, after the couple quit the royal family in early 2020.
The Daily Telegraph said on Saturday that the brothers had "agreed to draw a line under their public commemorations" at the 20th anniversary.
Spokespeople for both couples confirmed they would spend the day privately.
Diana's shock death aged just 36 saw a sea of floral tributes laid at the gates of her former London home, Kensington Palace.
Well-wishers are again expected to lay flowers at the palace, and near the entrance to the Paris road tunnel where the high-speed crash happened.
Diana remains widely revered as a fashion icon and for having cut through stuffy royal convention, particularly after her very public divorce.
She was also praised for charity work highlighting the scourge of landmines and encouraging a change in attitudes to people with HIV/AIDS.
In a sign of the enduring fascination with the royal, last weekend a sporty Ford Escort she once owned was sold for pound sterling737,000 ($864,000) at auction.
Her marital troubles will also feature in the latest series of the hit Netflix drama "The Crown" later this year.
Joe Little, managing editor at Majesty Magazine, said Diana had influenced her sons to be less formal than traditional royals.
"They are much more 'touchy feely' than they ever would have been," he said, pointing to William's hugs for England's women footballers after their Euro 2022 victory this month.
"William can see there is no need for much of the formality in royal life that existed when his father was of a similar age, the institution of the monarchy functions well without all that starchiness," he added.
Last week Harry, who has admitted to mental health struggles because of his mother's death, said he would "share the spirit of my mum with my family, with my children, who I wished could have met her".
"Every day I hope to do her proud," he told a fundraising dinner in the United States last week for his Sentebale charity, which supports young people with HIV.
"I want it to be a day filled with memories of her incredible work and love for the way she did it," he added.
William and Harry were just 15 and 12 when Diana died and provided the enduring image of her funeral, walking heads bowed behind her coffin.
Last year the brothers unveiled a statue of her at Kensington Palace on what would have been her 60th birthday.
They said they remembered "her love, strength and character -- qualities that made her a force for good around the world, changing countless lives for the better".
But a rift -- said to date back to soon after Harry began dating Meghan -- was barely concealed.
The pair were seen together briefly at the April 2021 funeral of their grandmother Queen Elizabeth II's husband Prince Philip.
They were apart too at the recent public celebrations marking the queen's 70 years on the throne.
William has angrily hit back at claims in a television interview from the couple of racism in the royal family.
Harry, who is expected to make more revelations in an upcoming memoir, has also said William and their father were trapped in a hidebound institution.
The couple are due at a series of charity events in Britain next week, staying near William and Kate on the sprawling Windsor estate.
There are reportedly no plans for the brothers to meet, nor for Harry to visit his ailing 96-year-old grandmother at her Balmoral retreat in Scotland.