25It must be recalled that, according to settled case-law, the letter of formal notice sent by the Commission to a Member State and the reasoned opinion issued by the Commission delimit the subject-matter of the dispute, so that it cannot thereafter be extended. The opportunity for the State concerned to submit its observations, even if it chooses not to avail itself thereof, constitutes an essential guarantee intended by the EC Treaty, adherence to which is an essential formal requirement of the procedure for finding that a Member State has failed to fulfil its obligations. Consequently, the reasoned opinion and the proceedings brought by the Commission must be based on the same complaints as those set out in the letter of formal notice initiating the pre-litigation procedure (see, in particular, Case C-191/95 Commission v Germany [1998] ECR I-5449, paragraph 55, and Case C-340/96 Commission v United Kingdom [1999] ECR I-2023, paragraph 36).
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