Short answer. It becomes mandatory in two situations: when directed to do so by the President, or when, upon complaint or otherwise, the Solicitor General or a public prosecutor has good reason to believe the usurpation, forfeiture, or unlawful corporate act can be established by proof.

What the law says

The Solicitor General or a public prosecutor, when directed by the President of the Philippines, or when upon complaint or otherwise he has good reason to believe that any case specified in the preceding section can be established by proof, must commence such action.

Rule 66, Section 2 — When Solicitor General or public prosecutor must commence action. Read the full provision →

Not always discretionary

Ordinarily one might assume the Solicitor General has broad discretion whether to sue, but this section makes commencing a quo warranto action mandatory in defined circumstances rather than purely optional. The word used is "must," not merely "may," once either triggering condition is present. This distinguishes it from the general run of prosecutorial or litigation discretion, where the decision to file or not file typically rests entirely with the officer's own judgment and assessment of the case. Once one of the two statutory triggers is present, that discretion narrows sharply into a legal duty the officer cannot simply decline to perform. An officer who has good reason to believe usurpation, forfeiture, or an unlawful corporate act can be proven, yet who simply declines to act, is not exercising discretion the rule actually grants — the discretion ends once the triggering condition is satisfied.

The two triggers

The first trigger is a direct instruction from the President of the Philippines. The second is more independent: upon complaint or otherwise, the Solicitor General or a public prosecutor forms a good-reason-to-believe that one of the situations covered by the preceding section — usurpation, forfeiture, or unlawful corporate action — can actually be proven. Either trigger standing alone is sufficient; the officer does not need both a presidential directive and an independent finding of good reason before the duty to file attaches — satisfying just one condition alone already triggers the mandatory obligation to act. Because the second trigger can arise "upon complaint or otherwise," the officer's duty to evaluate whether good reason exists can be set in motion by a private citizen's complaint just as readily as by the officer's own independent investigation.

Two officers, one duty

The obligation applies equally to the Solicitor General and to a public prosecutor, giving both offices standing and a corresponding duty to act once either triggering condition is met, rather than reserving the mandatory duty to only one of them. This dual coverage means the appropriate officer at either the national or the local level can be the one who actually commences the action, depending on where the alleged usurpation or forfeiture arises and which officer first learns of it. Naming both offices in the same mandatory duty also prevents the obligation from being avoided simply because the matter first surfaces before the local prosecutor rather than the Solicitor General, or the other way around.

Related provisions

Note. Statute text quoted on this page is reproduced from the official enactment and is linked to the full provision. The explanation around it is general legal information from Vivas & Nobles Law Office, not legal advice. Whether it applies to your situation depends on facts only a lawyer reviewing them can assess.