The long-running legal battle between the two mobile phone giants has ended in the High Court with Mr Justice Lewison rejecting O2's complaints that the campaign for 3's ThreePay service was misleading, infringed its bubble trademark and discredited O2's services.
He told the court: "Within the confines of the Comparative Advertising Directive, advertisers should in my judgment be permitted to do what is needed to make the comparative advertising effective.
3 owner Hutchison 3G has described the ruling as "ground breaking" and the first real legal test of how trademark laws should interact within the rules governing comparative advertising.
Lewis Silkin, acting for Hutchison 3G, said the ruling could have far-reaching implications for brand owners.
He saod: "The judgment upholds the important principle that honest comparative advertising is in consumers' interests and advertisers should be given reasonable leeway to engage in robust but fair comparisons.
"It is a judgment which is of great interest and significance to brand owners, their agencies and advisers throughout Europe."
A spokesman for O2 said a decision on whether to appeal against the decision had not yet been reached.
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