The European Court of Justice is to consider the extent to which European law allows comparative advertising in the latest round of the battle between rival mobile phone operators O2 and 3.

In its ruling, handed down on December 5, the UK Court of Appeal asked the ECJ to consider three questions.

Lord Justice Jacob, one of the three judges hearing the case, said: "If we decided matters for ourselves, whichever way we went, and whether or not we granted leave to appeal to the House of Lords, a reference in the end would be inevitable. So it is better to refer now rather than allow the delay and expense which would otherwise be involved."

O2 accused 3 of infringing its trade marks by using similar imagery to O2's brand bubble image in a 2004 comparative advertising campaign. 3's advert featured white bubbles on a black screen, similar to O2's protected imagery of blue bubbles on a dark-blue background, with a voice-over saying O2's prices are more expensive than those offered by 3's service.

In March, the High Court ruled that 3's adverts for its pay-as-you-go service complied with advertising rules and did not cause confusion with O2.

Now the Court of Appeal is asking the ECJ to consider:

First, where a defendant in the course of trade uses a sign in a context purely for the purpose of comparing the merits (including price) of his goods or services with those of the trade mark owner and in such a way that it cannot be suggested that the essential function of the trade mark to guarantee the trade mark as an indication of origin is in any way jeopardised, can his use fall within either (a) or (b) of Article 5(1) of Directive 89/104?

Second, where a defendant uses, in a comparative advertisement, the registered trade mark of another, in order to comply with Article 3a of Directive 84/450 as amended, must that use be 'indispensible' and, if so, what are the criteria by which indispensability is to be judged?

Third, in particular, if there is a requirement of indispensability, does that requirement preclude use of a sign so similar to the registered trade [mark] as to be confusingly similar to it?