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However Aldi, said comparative advertising was a "well-established principle".

Aldi said comparative advertising was a "well-established principle".

Ross Petty, a professor of marketing law at Babson College, says the FTC endorsed comparative advertising because it preferred brands to compete with one other than with an unnamed competitor.

"Comparative advertising encourages product improvement and innovation, and can lead to lower prices in the marketplace," the statement read.

In Europe, comparative advertising is also legal although much more tightly regulated, says Petty of Babson College.