To answer the question, it must first be recalled that the system of protection established by Directive 93/13 is based on the idea that the consumer is in a weak position vis-à-vis the seller or supplier as regards both his bargaining power and his level of knowledge, which leads to the consumer agreeing to terms drawn up in advance by the seller or supplier without being able to influence the content of those terms (Case C-453/10 Pereničová and Perenič [2012] ECR, paragraph 27, and Case C-472/10 Invitel [2012] ECR, paragraph 33).

In view of that weak position, Directive 93/13 prohibits, first, in Article 3(1), standard terms which, contrary to the requirement of good faith, cause a significant imbalance in the parties' rights and obligations arising under the contract, to the detriment of the consumer.

Directive 93/13 imposes, secondly, in Article 5, an obligation on the seller or supplier to draft terms in plain, intelligible language.

A standard term which allows such a unilateral adjustment must, however, meet the requirements of good faith, balance and transparency laid down by those directives.

With respect, in the first place, to the information to be given to the consumer, it is clear that that obligation to make the consumer aware of the reason for and method of the variation of those charges and his right to terminate the contract is not satisfied by the mere reference, in the general terms and conditions, to a legislative or regulatory act determining the rights and obligations of the parties.

It is essential that the consumer is informed by the seller or supplier of the content of the provisions concerned (see, to that effect, Invitel, paragraph 29).

While from the point of view of point 2(b) of the annex to Directive 93/13 and point (b) of Annex A to Directive 2003/55 it is indeed for the supply undertaking to notify consumers in good time of any increase in tariffs and of their right to terminate the contract, that obligation, laid down for cases in which the undertaking actually wishes to exercise the right reserved to it to amend the tariffs, is in addition to the obligation to inform the consumer, before conclusion of the contract and in clear and intelligible terms, of the principal conditions of the exercise of that right of unilateral variation.

With respect, in the second place, to the consumer's right to terminate the supply contract he has concluded in the event of a unilateral alteration of the tariffs applied by the supplier, it is of fundamental importance, as the Advocate General says in substance in point 85 of her Opinion, that the right of termination given to the consumer is not purely formal but can actually be exercised.