Her remarks drew comparison with the infamous 1991 after-dinner speech by Gerald Ratner, who sent his eponymous jewellery firm into a spiral of decline when he said of its stock: "People say, 'How can you sell this for such a low price?' I say, 'Because it's total crap.'"
Dame Helen, 72, is the oldest of the French brand's celebrity ambassadors and has insisted that the advertisements featuring her are not digitally retouched to erase wrinkles.
She is the face of L'Oreal's 'Age Perfect' moisturiser range, and was signed as a brand spokeswoman in 2014 after their consumer research found she was perceived as "genuine, intelligent and glamorous, with looks that seem only to improve with the passing of time".
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She describes herself as a long-time fan of the French brand, and said: "It used to drive me crazy that the ads promoting skin products were using pictures of 15- and 16-year-old girls.
"As a 30-year-old, I used to look at that and think, what the f*** are you talking about? It was ridiculous. Pissed me off majorly. Advertisers are only just coming out of that, and it's taken them a long time."
Speaking at the Cannes event, Dame Helen also reflected on her career.
"I often feel I've lost my way in my aspiration to be an artist," she said in a speech reported by Campaign magazine.
"Of course, it's wonderful to have success, and the thing I love most about it is economic freedom. Coming from a background where there was no money - and if you didn't work, there was nothing to live on - to have bought my own house and to have never been in debt or on welfare is something I feel proud of.
"But there's a part of me that thinks I should have been a struggling artist."
Asked if she felt she had sold out, Dame Helen replied: "Yes, I do in many ways. But, I may add, blissfully and quite happily."
Fans who dream of seeing Dame Helen as a female James Bond will be disappointed. "I'm too old," she said.
"In my youth, that would have been great, of course. But that time was different; we could never even have imagined a woman playing that role."
A L'Oreal spokesman was not available for comment.
This story originally appeared on the Daily Telegraph and is reproduced with permission.