Certificate A1 Break in Posting: What Happens in 2026
When a posted worker returns to Poland for a full month during an ongoing assignment in Denmark, one question immediately surfaces: does the A1 certificate break in posting invalidate the document, or can the original certificate simply continue? The answer is not straightforward, and getting it wrong can expose both the worker and the sending employer to serious social security and tax consequences on both sides of the border.
Why the A1 Certificate Matters So Much
The A1 certificate, issued under EU Regulation 883/2004 on the coordination of social security systems, is the document that proves a posted worker remains covered by the social security legislation of their home country, in this case Poland, while temporarily working in another EU member state such as Denmark. Without a valid A1, Danish authorities and the Danish Labour Inspectorate, known as Arbejdstilsynet, may treat the worker as subject to Danish social security contributions from day one of the assignment, which can trigger significant back-payment demands.
For Polish sending companies, understanding the interaction between a posting interruption and the A1's validity is therefore not a bureaucratic detail. It is a core compliance obligation. If your workers are also subject to collective agreements in Denmark, the broader framework is worth reviewing in this guide on Overenskomst in Denmark: A Guide for Polish Agencies 2026.
Step 1: Check Whether the Interruption Is Considered a Break
Not every return to Poland resets the clock. EU administrative guidance, developed through the Administrative Commission for the Coordination of Social Security Systems, distinguishes between a genuine interruption of posting and a temporary absence. A worker taking annual leave, sick leave, or a short training period back in Poland does not automatically break the posting. However, a full month during which the worker performs work duties in Poland, receives assignments there, or is effectively re-integrated into the Polish workforce is treated very differently.
The key questions to ask are: Was the worker performing work in Poland during that month? Was the employment contract with the sending company continuously active, with no new contract signed? Was the worker still economically dependent on the same Polish employer throughout? If the answers point to a genuine resumption of Polish work activity, the original A1 certificate may be considered interrupted, and a new posting period, with a new A1 application, will be required when the worker returns to Denmark.
Step 2: Gather the Required Documents Before Acting
Before contacting ZUS, the Polish Social Insurance Institution, you need to assemble the documentation that tells the full story of the interruption. This typically includes the original employment contract, any amendment or annex signed before the return to Poland, payroll records showing where contributions were paid during the gap month, and a written statement from the employer explaining the reason for the worker's return. If the worker was on documented sick leave or statutory annual leave, those records become your primary evidence that the posting was merely suspended rather than terminated.
Polish employers should also keep in mind the obligations under the Kodeks Pracy, the Polish Labour Code, regarding documentation of secondments and changes to work location. Proper internal records are your first line of defence in any inspection by PIP, the National Labour Inspectorate in Poland.
Step 3: Determine Whether You Need a New A1 or a Correction
If ZUS concludes that the original posting was genuinely interrupted, you will need to apply for a new A1 certificate covering the resumed period in Denmark. This is done through the ZUS online platform, PUE ZUS, available at www.zus.pl. The application form is US-4, used for requesting an A1 under Article 13 or Article 12 of Regulation 883/2004 depending on your situation. Fill in the new start date of the posting carefully, as it must match the worker's actual return to Denmark, not the original assignment start date.
If, on the other hand, the documentation clearly shows the absence was leave-based and the posting was never truly interrupted, ZUS may issue a correction or confirmation letter. In either case, do not assume the original certificate remains valid without written confirmation from ZUS. Assumptions here are exactly what lead to compliance failures discovered months later during a Danish tax audit. Workers who have Danish tax questions relating to their period of posting should also consult the guidance on Danish Tax Refund for Polish Construction Workers 2026.
Step 4: Submit the Application and Notify the Danish Side
Once the application is submitted through PUE ZUS, the processing time can vary. For straightforward cases, ZUS typically issues the certificate within a few weeks, though complex cases involving posting interruptions may take longer. During this waiting period, the worker should not resume work in Denmark without either the new A1 or at minimum a written acknowledgement from ZUS that the application is under review.
On the Danish side, the sending company or the Danish client should be informed of the situation in writing. Arbejdstilsynet has the authority to request A1 documentation during site inspections, and presenting an expired or invalidated certificate is treated as a violation. Keeping the Danish contracting party informed also protects the sending agency under the terms of any service agreement, particularly if that agreement includes provisions about social security compliance. For context on how such agreements are typically structured, see the article on Terminating a Polish Agency Contract in Denmark 2026.
Step 5: Wait for the Result and Update Your Records
When the new A1 arrives, update your internal HR records immediately. The certificate number, validity dates, and the worker's personal identification details should be logged and a copy provided to the worker to carry on site. Under Danish rules, workers must be able to present social security documentation upon request during a workplace inspection.
Keep the original A1, the gap-period documentation, and the new A1 together in a single file. If an inspection by Arbejdstilsynet or a query from SKAT, the Danish Tax Agency, arises later, having a clean chronological paper trail is the most effective way to resolve it quickly. Information on SKAT obligations for posted workers can be found at skat.dk.
Common Mistakes to Avoid
The most frequent error is assuming that a return to Poland for any reason automatically preserves the original A1. It does not, and this assumption has caused genuine compliance problems for sending companies. A second common mistake is backdating the new A1 application to cover the gap period retroactively without ZUS approval. ZUS can issue retroactive certificates in certain circumstances, but this requires explicit justification and is not guaranteed.
A third mistake is failing to distinguish between the posting period under the A1 and the maximum posting duration allowed under EU Directive 96/71/EC on the posting of workers, as amended by Directive 2018/957. These are separate legal frameworks with separate clocks, and a break in posting may or may not reset the duration counter depending on how the interruption is classified.
The practical advice is this: as soon as a worker's return to Poland looks likely to last more than a few days and will involve active work duties there, contact ZUS proactively, document everything in writing, and do not allow the worker to resume the Danish assignment until the social security position is confirmed. A short administrative delay is far less costly than a retroactive reclassification of social security obligations stretching back months. The rules exist to coordinate systems across borders, and they work well when employers engage with them honestly and in good time. Official guidance on the coordination rules is published by the European Commission at ec.europa.eu.