The purchaser may award a contract without prior advertising or competitive tendering if only irregular tenders, as defined in article L. 2152-2, or only unacceptable tenders, as defined in article L. 2152-3, have been submitted. In such cases, the initial terms of the contract may not be substantially altered and the purchaser must only allow the tenderer or tenderers who have submitted tenders that comply with the requirements relating to the formal deadlines and procedures for submission to take part in the negotiations.