Legal Challenge Against IS 2025 Models: 9 Days to File

Key data

The official Corporate Income Tax models for fiscal years beginning in 2025 are being challenged in court. Prezero Iberia, SLU has filed administrative contentious proceedings 886/2026 before the National Court, Administrative Contentious Chamber, Second Section, against Order HAC/529/2026, which is the regulation that approves said declaration models, both for IS and for the Non-Resident Income Tax (IRNR).

The Resolution of 2 September 2026, published in the BOE on 9 September 2026, orders the summons of all interested parties. This means that any company or entity that considers it has a legitimate interest in the outcome of this challenge has a very narrow window of time to act.

What does this regulation establish?

The resolution published in the BOE does not resolve the merits of the case: it simply notifies all possible interested parties that a challenge is underway and that they may join the procedure if they wish.

The subject of the challenge is Order HAC/529/2026, which approves the official declaration models for Corporate Income Tax and Non-Resident Income Tax corresponding to fiscal years beginning in 2025. These are the forms that all obligated companies must use to file their annual declaration.

If the challenge succeeds in the future, it could have consequences on the validity or content of those official models. In that scenario, companies that have filed their appearance in the procedure will be able to defend their interests directly; those that have not done so will be excluded from the judicial resolution.

Economic and operational impact

The immediate impact is procedural, not economic: there are no new fees, penalties or direct costs arising from this resolution. However, the indirect consequences may be relevant for certain companies.

• If the challenge succeeds: the official IS 2025 models could be declared invalid or modified. Companies that have already filed their declaration using those models could be affected, and those that have not yet done so could face uncertainty about the valid format.
• Cost of filing appearance: it involves hiring specialized legal representation in administrative tax litigation. There is no fixed amount; it depends on the law firm and the complexity of each company's particular case.
• Cost of not filing appearance: if the challenge succeeds and the company has not filed its appearance, it will not have been able to influence the outcome or defend its specific interests in the procedure.
• Critical deadline: only 9 business days from 9 September 2026. Once expired, the door closes.

Companies with complex tax situations in IS 2025 —controversial deductions, disputed extraaccounting adjustments, or that have had specific difficulties with the models approved by Order HAC/529/2026— are those that should most consider this option with their tax advisor or tax attorney.

Who does it affect?

• All companies and entities obligated to file Corporate Income Tax for fiscal years beginning in 2025.
• Entities subject to Non-Resident Income Tax (IRNR) that operate through a permanent establishment and must declare using the models approved by Order HAC/529/2026.
• Companies that have had specific discrepancies or difficulties with the content or format of the official IS 2025 models.
• Consolidated groups and large companies with complex tax structures that may be especially affected by any changes to the models.
• Tax advisors and tax law firms that manage IS declarations for their clients and must assess whether any of them have a legitimate interest in the challenge.

Practical example

A medium-sized industrial company, with a fiscal year coinciding with the calendar year, that filed its 2025 IS declaration using the models approved by Order HAC/529/2026 and that had to apply a debatable criterion in one of the sections of the model (for example, in completing an adjustment to the accounting result), could consider that it has a legitimate interest in this challenge.

If its tax advisor assesses that Prezero Iberia, SLU's challenge could affect that specific section of the model, the company has 9 business days from 9 September 2026 to file its appearance in proceedings 886/2026 before the National Court, Second Section. To do so, it must act through a lawyer and attorney authorized to appear before that court.

If it does not act within that deadline, the procedure will continue without it and it will not be able to join later to defend its position.

What should companies do now?

1. Verify the deadline immediately: the 9 business day deadline from 9 September 2026 is non-extendable. Calculate the specific deadline with your legal advisor taking into account applicable non-business days.
2. Assess whether there is a legitimate interest: consult with your tax advisor whether your company had difficulties, discrepancies or controversial situations related to the models approved by Order HAC/529/2026 for IS 2025.
3. Decide whether to file appearance: if there is a legitimate interest, instruct a lawyer specialized in administrative tax litigation to assess the advisability of filing appearance in proceedings 886/2026 before the National Court, Second Section.
4. Act with authorized representation: filing appearance before the National Court requires a lawyer and attorney authorized to appear before that court. It is not a procedure that the company can do directly.
5. Monitor the progress of the challenge: even if you do not file appearance, follow the progress of challenge 886/2026. If it succeeds, it could have consequences on the validity of the models you have already used to file your 2025 IS declaration.

Official source

View complete regulation at official source

Notice: This article is for informational purposes only and does not constitute legal advice. For specific decisions, consult a qualified professional. Source: https://www.boe.es/diario_boe/txt.php?id=BOE-A-2026-18877