Naomi Watts's 'Diana' shredded by Brit critics

"Diana" star Naomi Watts would be wise to avoid the U.K. papers today.

As the biopic readies to release there on September 20, British critics are warning moviegoers that they won't like what they find.

The film, directed by Oliver Hirschbiegel, focuses on the last two years of Diana's life, when she was in a relationship with heart surgeon Hasnat Khan. Watts was so anxious about how the film would be received, she initially turned down the job - twice.

Early reviews suggest she had a right to be worried.

The Guardian has accused Hirschbiegel of killing Princess Diana, who died in a car wreck in 1997, all over again with the film.

"This is due to an excruciatingly well-intentioned, reverential and sentimental biopic about her troubled final years, laced with bizarre cardboard dialogue," the review begins. "Is this film an MI5 plot to blacken Diana's name and make her look plastic and absurd? The movie isn't so much Mills & Boon as a horrendous 'Fifty Shades of Grey' with the S&M sex taken out – and replaced with paparazzi intrusion and misunderstood charity work."

The Telegraph was similarly unimpressed, calling "Diana" "hardly fascinating," free of "new facts about the Princess’s life. And it certainly doesn’t explain her complexity or contradictions. That would take a different, better film altogether."

The U.K.'s Mirror spoke its distaste more plainly: "Diana can only be described as a fabulously awful film," the review critiques. "Hierschbiegel should know better," and Watts, "despite a peroxide hair-job ... looks, sounds and acts nothing like the Princess of Wales. Wesley Snipes in a blonde wig would be more convincing."

Screen Daily agreed that the project is "plodding," a pace made all the more excruciating by "some truly dire dialogue." (The script was written by Stephen Jeffreys, who adapted it from the book "Diana: Her Last Love" by Kate Snell.)

There was one critic on "Diana's" side: The London Evening Standard, "fully expecting to hate it," but if viewers adjust their expectations they might wind up liking it.

"(T)his is a film, not a documentary. Two hours of theatre – a piece of entertainment. Nothing more or less," the review says. "This film may not be wholly accurate, there may be some gaping holes in the script and it may fall down on authenticity and Watts may not be the perfect carbon copy of the late Diana – but as a charming, impossible but ultimately tragic love story it works."

But despite the criticism and her own hesitation, Watts, 44, does believe that the story needed to be told.

"I was a little bit torn, but once I'd said no, I wasn't completely at peace. Sometimes when you say no, you feel free, but it just wasn't the case," Watts told the Sunday Times. "There was something very intriguing about this woman's life. It was extraordinary ... I liked the idea that there was this fame—no one at that level can really survive it, I think. It's a very dark thing, but it's kind of true."