43The obligation on the contracting authority to comply with the principle of equal treatment of tenderers, which seeks to encourage the development of healthy and effective competition between undertakings participating in a public procurement procedure, and which lies at the very heart of the EU rules on public procurement procedures, implies, in particular, that tenderers must be on an equal footing both when they formulate their tenders and when those tenders are assessed by that contracting authority. The principle of equal treatment and the obligation of transparency thus preclude any negotiation between the contracting authority and a tenderer during a public procurement procedure, which means that, as a general rule, a tender cannot be amended after it has been submitted, whether at the request of the contracting authority or at the request of the tenderer (see, to that effect, judgment of 11 May 2017, Archus and Gama, C‑131/16, EU:C:2017:358, paragraphs 25 and 27 and the case-law cited).
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