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Lead actress Naomi Watts has already defended her involvement in the controversial film, which follows Diana's romance with London-based Pakistani surgeon Dr Hasnat Khan.
But within hours of the premiere, a string of merciless reviews in the British press shattered the party spirit.
The Times praised Watts for doing "her level best with a squirmingly embarrassing script" but concluded that the film was still "atrocious and intrusive".
"Poor Princess Diana," wrote Guardian critic Peter Bradshaw.
"I hesitate to use the term 'car crash cinema'. But the awful truth is that 16 years after that terrible day in 1997 she has died another awful death."
The Daily Telegraph gave the film two stars -- one more than both the Guardian and Times -- but was also withering in its assessment.
"What's the point of Diana?" reviewer Davis Gritten asked rhetorically.
Based on Kate Snell's 2001 book, Diana: Her Last Love, the film suggests Diana started dating Dodi Fayed to make Khan jealous.
Diana and Fayed died when the Mercedes-Benz in which they were travelling slammed into a pillar in a Paris tunnel while being pursued by press photographers.
Diana and heir to the British throne Prince Charles divorced in 1996 after 15 turbulent years of marriage which produced two sons, Princes William and Harry.
Watts, dressed in a figure-hugging white gown, was joined on the red carpet in London's Leicester Square by British-Indian actor Naveen Andrews, who plays her on-screen lover.
The British-Australian actress who came to prominence in Mulholland Drive in 2001, admitted she’d taken a risk by accepting the role of the "People's Princess".
Asked if she felt the film would offend Diana's sons, she told BBC TV, "Hopefully if they get to see the film, they will feel that we have done it in a respectful and sensitive way.
"We try to honour the depiction of her character in the best possible way."
The film has been largely ignored by the royal family.
Its producer, Robert Bernstein, claims the royals gave some help in allowing filming in Kensington Gardens, where Diana used to go jogging, but a spokesman for the royals told AFP the area was not under its jurisdiction.
Some critics have noted that Watts bears little physical resemblance to Diana: she had to wear a prosthetic nose for the film.
She’s the only established movie star in the film. Andrews is best known for his role in the TV series Lost although he also played Juliette Binoche's love interest in The English Patient 17 years ago.
Khan, who still works in Britain, has described the film as "completely wrong" and said he didn’t intend to see it.
He’s never spoken of his relationship with Diana, meaning the film-makers had to imagine the scenes between him and the princess. -Sapa 
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