367. Many EDR systems adopt the principle of irrevocability of all those acts and decisions of the electoral authority that have not been challenged in a timely fashion within a period specified in law (see paragraph 217). It makes it impossible to question the validity of a particular electoral action or decision at a later stage once it has become firm. For example, in several countries, like Mexico, it is not legally permissible for an irregularity committed during the election campaign to be raised as grounds for annulling the election during the stage of announcing the results if it was not opposed by the person or party affected during the stage of preparing the election. 368. It is important to note that this principle only operates when the alleged irregularity was susceptible to challenge at the time. If no means of challenge is provided for at that earlier stage there would be grounds for considering it proper to challenge it, for example, at the results stage. Hence, the election laws should establish specific and systematic deadlines for challenges to certain actions or decisions (e.g. those related to the electoral register or the nomination of candidates), and for the EDRBs to issue their decisions on these challenges. 369. Once started, an electoral process cannot, in general, be halted because timely renewal of the representative organs of government depends on it. It is good practice not only that all challenges must be resolved in a timely fashion, but also that the fact of a challenge having been filed does not suspend the effects of the electoral action or decision challenged. This helps avoid the EDR system being used as a mechanism for blocking the proper unfolding of an electoral process. An alternative is to defer all challenges to the electoral results period, in order to ensure that the electoral process is not interrupted or encumbered by long judicial procedures. The potential disadvantage of this approach is that any very serious irregularity during the electoral process that affected the result could only be rectified by annulment of the election, because it would be too late for any other corrective measure to have any effect. 370. There is a distinction between those acts and decisions that have an impact on the unfolding of the electoral process and those which, strictly speaking, fall outside its scope, in which case the time periods for filing a challenge and resolving it may vary. Some types of challenge are more complex and so may require more time to collect the information needed for their processing, consideration and resolution (for example those related to oversight of political parties’ resources). To the extent that they do not have an impact on the electoral process itself, the time periods for hearing these can be longer. The possibility may be considered of establishing by law that the respective decisions, administrative and judicial, should not be made during the electoral process so as not to ‘contaminate’ it. 371. In the case of alleged electoral crimes or offences, care should be taken to ensure that the investigation and any criminal prosecution are undertaken 126