The ad, by Kindred Agency, featured a nervous-looking man sitting on a sofa with a glass of beer, next to a woman wearing a figure-hugging dress with the label still attached.
Text from a speech bubble coming from a pint of beer read "take courage my friend".
Three members of the public complained that the poster implied the beer would give the man confidence to either make negative comments on the woman's appearance or take advantage of her.
Wells & Young's Brewing Company argued that its customer research had not raised any suggestion that the beer would give the man courage.
It said it had taken legal advice to ensure that advertising rules were not broken.
But the Advertising Standards Authority ruling said: "…we considered that the combination of the text and the image of the man with an open beer can and half-empty glass of beer was likely to be understood by consumers to carry the clear implication that the beer would give the man enough confidence to tell the woman that the dress was unflattering."
It ruled that the poster must not appear again in its present form.