"While the poster is magnificent, the photograph has clearly and deplorably been airbrushed to thin the actress's thighs," said Telerama magazine.
Similar reaction has come from other papers and users of social media.
The picture of Cardinale swirling her skirt was taken on a Rome roof in 1959.
"Claudia Cardinale dropped a dress size in one swirl," said Liberation newspaper.
And on Twitter, reaction was similar.
"Why the need to alter Claudia Cardinale's body so dramatically for Cannes 2017 official poster?," said Anna Rose Holmer.
But the festival has defended the poster.
Director Thierry Fremaux said it had been "very well received", reported AFP.
And on his Twitter page he posted supportive messages.
"What is this ridiculous outcry over the Cannes poster? All photos used for advertising are retouched in one way or another," said Jean-Paul Salome.
'Honoured and proud'
Cardinale herself, now 78, has raised no objection. In a statement issued by the festival, she said she was proud to have been chosen for the poster.
"I am honoured and proud to be flying the flag for the 70th Festival de Cannes and delighted with this choice of photo.
"It's the image I myself have of the festival, of an event that illuminates everything around.
"That dance on the rooftops of Rome... reminds me of my origins, and of a time when I never dreamed of climbing the steps of the world's most famous cinema hall."
The actress featured in classic movies of the 1960s and 70s, including The Leopard with Burt Lancaster and 8 1/2 by Frederico Fellini. She continues working now, her latest project being the Italian drama Piccolino Bella [Beautiful Little Girl).
This year's festival runs from 17-28 May and the official selection of movies will be announced on 13 April.
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