French media poured scorn on the festival for seemingly tampering with a photograph of Cardinale swirling her skirt on a Rome roof in 1959.
"Claudia Cardinale dropped a dress size in one swirl," said the left-leaning Liberation, while the culture magazine Telerama questioned why it was necessary to retouch the famously sexy star when she was in her heyday.
"While the poster is magnificent, the photograph has clearly and deplorably been airbrushed to thin the actress's thighs. What a pity," it said.
There was a similarly hostile reaction on social media, with one Twitter user saying, "If even Claudia Cardinale cannot represent beauty without being retouched, we really are in trouble."
Others bemoaned the "dictatorship of thinness" or claimed that "Cardinale wasn't thin enough for Cannes".
Claire Serre-Combe, of the French women's group Osez le feminisme! ("Dare to be Feminist"), said it was a scandal that Cardinale who was "thin and magnificent in the original photo", should have to "lose kilos" for the poster.
But the festival's director Thierry Fremaux told AFP that the poster had been "very well received".
'Happy about the choice'
The actress, now 78, said in a statement released by the festival that she was honoured to feature on the poster for its 70th anniversary year.
"I am very happy about the choice of the photo," she said.
"That dance on a roof in Rome... reminds me of a time when I could never have imagined climbing up the red-carpeted steps of the most celebrated" film festival in the world.
Tunisian-born Cardinale starred in some of the greatest European films of the 1960s and 1970s including Federico Fellini's "8½" and Luchino Visconti's "The Leopard".
The furore comes as Fremaux is whittling down the line-up for the festival, which runs from May 17 to 28.
Sofia Coppola's US Civil War story "The Beguiled", starring Colin Farrell as a wounded soldier who is given refuge in a girl's boarding school, is hotly tipped to make the main competition.
Coppola's previous hits "Virgin Suicides" and "Marie Antoinette" also premiered on the Croisette, and her new film comes with the starry trio of Nicole Kidman, Elle Fanning and Kirsten Dunst.
Brad Pitt, Matt Damon
Fellow US indie director Todd Haynes is also among those thought to have made the cut with "Wonderstruck", his time-shift story of two deaf children starring Julianne Moore and Michelle Williams.
Speculation is also swirling around "War Machine", the big-budget Afghan war story starring Brad Pitt.
Matt Damon could end up being in two films at the festival, Alexander Payne's sci-fi comedy "Downsizing" and George Clooney's new family drama "Suburbicon".
There has been much talk too of a special screening of Cannes favourite David Lynch's revival of his cult television series "Twin Peaks".
There are also likely to be films from Palme d'Or winners Roman Polanski -- "After a True Story" starring Eva Green -- and the unsparing Austrian director Michael Haneke tackling the migrant issue in "Happy End", which reunites him with French actress Isabelle Huppert.
Another winner of the festival's top prize, the Franco-Tunisian Abdellatif Kechiche ("Blue is the Warmest Colour") is expected to return with "Mektoub is Mektoub" (Fate is Fate).
French Oscar winner Michel Hazanavicius ("The Artist"), whose film on the cinema revolutionary Jean-Luc Godard is called "Redoutable", is also likely to be back this year.