Legal Challenge Against IS 2025: What Companies Can Do Now

Key data

The official models of the Corporate Income Tax for fiscal year 2025 are being challenged in court. The Spanish Association of Tax Advisors (AEDAF) has filed administrative litigation 863/026 before the National Court against the Order HAC/529/2026, which is the regulation that approves these forms. The Resolution of September 2, 2026 from the General Technical Secretariat publishes the mandatory summons so that any interested party can appear in the proceedings.

The deadline is short: 9 days from publication in the BOE (September 9, 2026), which sets the approximate deadline at September 19, 2026. After that deadline, it is no longer possible to join the proceedings as an interested party.

What does this regulation establish?

The published resolution does not resolve the merits of the case: its sole purpose is to publicize the judicial summons so that interested parties can appear. The underlying challenge attacks the Order HAC/529/2026, which is the ministerial order that each year approves the declaration models for Corporate Income Tax and Non-Resident Income Tax (IRNR) corresponding to fiscal years beginning in 2025.

AEDAF, as a representative association of tax advisors in Spain, considers that this order has deficiencies or irregularities sufficient to be annulled or modified by court. Although the specific content of the grounds for the challenge is not detailed in the summons resolution, the fact that AEDAF interposes it—an organization with extensive experience in tax litigation—gives practical relevance to the proceedings.

A potential favorable court ruling could affect:

• The official declaration forms for IS 2025 (models approved by Order HAC/529/2026)
• The filing deadlines established in that order
• The declaration procedures in force for fiscal year 2025

Economic and operational impact

At this time, the challenge does not suspend the obligation to file IS 2025 nor does it alter the deadlines in force. Companies must continue to comply with their tax obligations in accordance with current regulations while the judicial proceedings continue.

However, the potential impact is relevant in two scenarios:

• If the court partially upholds the challenge: certain sections of the models could be modified, which would require filing amended or corrected declarations. This would generate additional administrative and advisory costs.
• If the court fully upholds the challenge: the annulment of Order HAC/529/2026 would require the Administration to approve new models, with the consequent impact on deadlines and procedures for all affected companies.

For companies with permanent establishments in Spain and for entities under income attribution regime established abroad, the scope of the challenge also directly affects them, since Order HAC/529/2026 also regulates the IRNR models applicable to these cases.

Appearing in the proceedings allows a company or entity to be notified of all judicial resolutions and, if applicable, to defend its procedural position. Not appearing does not exempt from the effects of the judgment, but it does prevent active participation in the proceedings.

Who does it affect?

• Commercial companies required to file Corporate Income Tax for the fiscal year beginning in 2025
• Non-profit entities subject to IS (foundations, associations, cooperatives...)
• Permanent establishments in Spain of non-resident entities, subject to IRNR with models approved by the same Order HAC/529/2026
• Entities under income attribution regime established abroad with presence in Spain
• Tax advisors and management firms that file declarations on behalf of their clients and may be affected by changes in models or procedures
• CFOs and financial directors of companies with pending or already filed IS 2025 declarations

Practical example

A medium-sized company with fiscal year-end in December 2025 has already filed its IS 2025 declaration using the models approved by Order HAC/529/2026. If the National Court upholds AEDAF's challenge and annuls or partially modifies that order, that company could be required to review its declaration and, depending on the scope of the judgment, file an amended or corrected declaration.

If that same company decides to appear now in the judicial proceedings (before approximately September 19, 2026), it will be notified of each court resolution and, if it deems appropriate, can file arguments defending its specific interests. To do so, it will need the assistance of a lawyer qualified before the National Court.

Conversely, a company that does not appear will also be bound by the final judgment, but without having been able to influence the proceedings or be directly informed of its progress.

What should companies do now?

1. Consult urgently with the tax advisor (before September 19, 2026) to determine whether it is advisable to appear in challenge 863/026 before the National Court. The 9-day deadline from BOE publication is non-extendable.
2. Evaluate specific exposure: if the company has IS 2025 declarations already filed or pending filing, analyze which sections of the models could be affected by a potential favorable ruling on the challenge.
3. Do not suspend or delay filing IS 2025: the challenge does not suspend the tax obligation. Continue complying with current deadlines to avoid surcharges or penalties.
4. Designate legal representation if you decide to appear: proceedings before the National Court require a qualified lawyer and, if applicable, a qualified attorney.
5. Monitor the proceedings: even if you do not appear, the company should be alert to the court ruling, as a favorable judgment could require subsequent actions (amended declarations, new models, etc.).

Official source

Consult complete regulation in 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-2026-18876