From January 1, 2026, all electronic invoices in Portugal will have a qualified electronic signature or a qualified seal. This is a legal requirement that marks a turning point in the reliability, traceability, and validity of invoices and tax documents. For a CFO operating in Portugal, this requirement has an impact on internal processes, integrations, document security, audits... In other words, on the entire business. Given its importance, at easyap we explain what will change in 2026, what ATCUD is, how this qualified electronic signature is implemented, and the risks to avoid in this regulatory transition.
What will change in 2026? Summary for CFOs
The first thing to know about the new regulation on electronic invoicing in Portugal and digital signatures is how it will be applied and who it affects:
1. Legal obligation
- From January 1, 2026, all invoices will have to include an electronic signature or a qualified electronic seal.
- Until December 31, 2025, unsigned PDF invoices remain valid.
- The measure guarantees the authenticity and integrity of tax documents.
2. Scope of application
The obligation covers:
- B2B billing.
- B2C billing.
- Billing to the Administration (B2G).
- Important tax documents circulated in PDF format (credit and debit notes, receipts, etc.)
None of this changes anything with regard to the two mandatory elements that were already required by law to appear on electronic invoices. These are:
- ATCUD code, which is the Unique Document Identifier, as we will explain later.
- Verifiable QR code.
Both will continue to be necessary and will now coexist with the qualified signature or seal.
Key concepts for understanding digital signatures in Portugal
To be more precise, let's analyze the three most important aspects of this new regulation.
Qualified electronic signature
It is the most advanced form of digital signature, recognized by eIDAS, created using a qualified digital certificate issued by an accredited provider. Its legal value is equivalent to a handwritten signature.
Qualified electronic seal
Identifies the company (not the individual). It is ideal for automated mass billing processes and guarantees the integrity and authorship of the document.
ATCUD
It is the Unique Document Identifier, which was already mandatory for all invoices and tax-related documents. Now, it must be generated from certified software, and each document requires a unique one per series.
This entire legal framework is set out in Law No. 45-A/2024. However, the Tax Authority often publishes technical details, so it is advisable to follow official communications.
Real operational impact: what will change in your company
For all of the above reasons, the transition to mandatory digital signatures in Portugal entails fundamental internal changes. For companies that have not yet begun the process, adaptation will have to be immediate. Therefore, the following four aspects must be taken into account:
1. Billing
Your system must:
- Generate invoices with ATCUD.
- Apply qualified electronic signature or qualified seal.
- Integrate with your ERP or POS.
- Integrate with the qualified certificate provider.
- Have strict SLAs for cloud signing, with:
- HSM (Hardware Security Module).
- Audit of each transaction.
- Revocation control.
- Reliable time stamping.
2. Certificates and custody
- The certificate must be issued by an accredited provider under eIDAS.
- It could be:
- From individuals (qualified signature).
- From entities (qualified seal).
- Key custody must be in HSM or certified cloud providers.
- Do not store passwords on local devices without control and encryption.
3. Internal control and auditing
- Complete record of all signatures applied.
- Exportable audit logs.
- Evidence of signature (visible or hidden).
- Preservation system that guarantees immutability.
- Document retention compliant with Portuguese regulations.
4. Costs and governance
Compliance with all these aspects, at the same time, entails the following costs and actions:
- Generate qualified certificates.
- Costs per stamp or signature.
- Costs of integrating an ERP and a billing module.
- HSM custody costs.
- Training and process updates.
- Assign roles:
- Tax compliance officer.
- Technical Manager (IT/Infrastructure).
Example of what mandatory digital signatures entail
To better understand how to comply with mandatory digital signatures in Portugal, let's look at a use case:
Company: Lusitania S.L. (manufacturing)
- Headquarters: Spain.
- Subsidiary: Portugal.
- Volume: 45,000 invoices per year.
Solution adopted
- The corporate ERP incorporates a local Portuguese module.
- The system automatically requests a qualified seal via API from a certified cloud provider.
- The invoice is generated in XML-PT/CIUS-PT, the ATCUD is added, the QR code is created, and only then is the stamp applied on the server.
Result
- Invoices are issued with a qualified seal and are fully valid from January 1, 2026.
- Complete audits of all invoicing are provided.
- All operational risk is reduced.
Minimum technical requirements that your billing software must meet
Therefore, and in a context where electronic invoicing in Portugal is already highly regulated, your technological solution must support:
- Automatic generation of ATCUD.
- Qualified electronic signature or qualified seal.
- Key custody and rotation in HSM.
- APIs for integration with ERP and back-office systems.
- Batch signing capability in batch and in real time.
- Exporting logs and evidence for auditing purposes.
- Compatibility with CIUS-PT, signed PDF, and QR.
Risks to your business if you don't adapt in time
The worrying and dangerous thing about not complying with the regulations in January 2026 is that you expose yourself to certain risks:
- Penalties for issuing documents that do not comply with the new legal provision.
- Loss of probative value in disputes with suppliers or in audits.
- Operational errors due to having to redo entire batches of invoices.
- Blocking in ERP systems if there is no good integration for automatic signing.
- Cost overruns due to having to implement emergency solutions or work with unforeseen suppliers.
Best practices for secure implementation
We will also reveal best practices for complying with mandatory digital signatures in Portugal:
- Use a qualified seal for bulk invoicing.
- Use the qualified signature on sensitive or high-value invoices.
- Always work with HSM or certified cloud services.
- Periodically audit the traceability of signatures and ATCUD.
- Make a pilot series before going into production.
- Establish a strict policy for recovering and reissuing invoices.
To make everything clearer and in a unified visual format, here is a table with risks, best practices, and how easyap helps you comply with mandatory digital signatures if you operate in Portugal.
| Risk | Best practice | What does easyap offer? |
| Invalid invoices and penalties | Always use a qualified seal or signature. | Signature/stamp integrated with automatic ATCUD |
| Operational shutdown | Automate signing and validation | Batch and real-time signature engine |
| Lack of traceability | Use of logs, timestamps, and legal retention | Audit and secure custody portal |
| Risk of fraud or manipulation | Custody in HSM and encryption | eIDAS-certified cloud HSM |
| Last-minute urgent costs | Plan pilot by series | Guided onboarding + expert support |
Why easyap is the best option for complying with digital invoicing
Adopting mandatory digital signatures in Portugal affects the technological architecture, tax compliance, and financial processes of the entire organization. In this scenario, easyap is the ideal partner for companies operating in Portugal.
Our electronic invoicing platform is equipped to manage ATCUD, QR, electronic signatures, qualified seals, and legal storage from a centralized environment. This allows you to maintain financial flows without interrupting your ERP.
Specifically, easyap automates signing through a secure API, with HSM custody, time stamping, and full auditing. This ensures compliance with Portuguese regulations and eliminates manual tasks, prevents errors, reduces costs, and provides traceability for audits.
In addition, we combine technical expertise and tax knowledge, ensuring a guided, fast, and risk-free implementation. For a CFO, this translates into less complexity, lower exposure to penalties, and a smooth transition to the 2026 mandate. Contact us and we will demonstrate this to you.
