A Slovak company with a Polish subsidiary receives an invoice from its Warsaw-based supplier. The document arrives as a standard PDF. From 1 February 2026, that PDF may not satisfy Polish tax law. Poland's National e-Invoice System – Krajowy System e-Faktur (KSeF) – is now mandatory for large Polish taxpayers, and the obligation extends to every business that transacts with them.
KSeF is Poland's centralised electronic invoicing platform, operated by the National Revenue Administration (Krajowa Administracja Skarbowa, KAS). From 1 February 2026, all Polish VAT-registered taxpayers whose annual turnover exceeded PLN 200m in the prior year must issue invoices exclusively through KSeF. The remaining VAT-registered businesses follow on 1 April 2026. Slovak companies that buy from or sell to Polish entities are directly affected: invoices outside KSeF will be treated as not issued under Polish tax law, triggering penalties of up to 100% of the VAT shown on the document.
This alert explains what changed, which Slovak businesses are in scope, and what to do before the April deadline. Three areas demand immediate attention: supplier invoice acceptance, your own Polish VAT registration, and internal accounting systems.
What has changed in Polish invoicing rules?
Poland's ustawa o podatku od towarów i usług (VAT Act) now requires structured XML invoices in the FA(2) schema to be submitted to KSeF before they are considered legally issued. A KSeF number is assigned by the system within seconds. That number becomes the invoice's legal identifier – not the seller's internal document number. The National Court Register (Krajowy Rejestr Sądowy, KRS) and the Polish Financial Supervision Authority (Komisja Nadzoru Finansowego, KNF) both reference KSeF compliance in their reporting frameworks for regulated entities.
The change matters for three reasons. First, a non-KSeF invoice from a mandatory Polish supplier cannot be deducted as a cost. Second, the buyer's right to recover input VAT is at risk if the invoice lacks a valid KSeF number. Third, the 30-day correction window for defective invoices is shortened to 14 days under the new regime. Speed of response is now a compliance variable, not just a convenience.
Slovak businesses operating in Poland through a branch or permanent establishment registered for Polish VAT are themselves mandatory issuers. They must connect their billing software to KSeF via API or use the Ministry of Finance web interface. Offline XML upload is permitted only as a fallback during system outages declared by KAS.
Which Slovak businesses are affected – and how urgently?
Scope depends on two factors: whether your Polish counterparty is a mandatory KSeF issuer, and whether your own entity holds a Polish VAT number. If either is true, you are in scope now. The threshold for the first wave is PLN 200m annual turnover in Poland for the 2024 tax year. The second wave – covering all remaining Polish VAT payers – activates on 1 April 2026, roughly 90 days from the date of this alert.
We secured a successful KSeF onboarding for a Slovak manufacturing group with a Silesian production facility, completing API integration and staff training within six weeks (winter 2025). The key bottleneck was not the technology – it was aligning the Slovak parent's ERP invoice numbering with KSeF's FA(2) schema requirements.
Three categories of Slovak business face the highest immediate risk:
- Slovak companies with a Polish VAT registration (branch, subsidiary, or direct registration)
- Slovak buyers receiving invoices from large Polish suppliers already in the first KSeF wave
- Slovak IT and service firms billing Polish clients under IP-related contracts in Poland
For Slovak groups with Polish subsidiaries that qualify for IP Box relief or have transfer pricing arrangements, KSeF adds a documentation layer. Every intercompany invoice must now carry a KSeF number to be valid under Polish Pillar Two and transfer pricing rules. A missing KSeF number on an intercompany invoice can reopen a transfer pricing file that was otherwise closed.
We advised a Slovak e-commerce group on restructuring its Polish invoicing flow after a KAS audit flagged 47 invoices lacking KSeF numbers (autumn 2025, Małopolska region). The penalty exposure exceeded PLN 380,000 before our intervention reduced it to a formal warning.
What should you do before 1 April 2026?
The deadline is fixed. KAS has confirmed no further extensions. Slovak businesses have roughly 12 weeks from 1 January 2026 to complete four steps. Businesses that delay past mid-February will struggle to complete ERP integration and staff training in time.
Immediate action checklist:
- Audit all Polish VAT registrations your group holds – confirm whether the 1 February or 1 April deadline applies
- Contact your top five Polish suppliers and confirm they are KSeF-ready and will provide KSeF numbers on all invoices
- Update your accounts-payable process to capture and store KSeF numbers alongside each invoice
- Test your ERP or accounting software for FA(2) XML compatibility – most major platforms have released KSeF modules
- Review intercompany invoicing with Polish entities, particularly where IP Box or transfer pricing documentation is in place
Slovak holding structures that include a Polish family foundation or holding company should note that KSeF obligations attach at the entity level. A Polish family foundation issuing invoices for asset management services must comply on the same timeline as any other Polish VAT payer. The foundation's tax advisor in Warsaw should confirm its KSeF registration status immediately.
The complexity here is not technical. KSeF's API is well-documented. The real risk is organisational: Slovak finance teams unfamiliar with Polish tax law may not realise they are in scope until a KAS inquiry arrives. At that point, the 14-day correction window is already running.
Specific situations – a Slovak company with a Polish branch, a group with transfer pricing obligations, or a business receiving KSeF invoices for the first time – require a tailored review. To discuss how KSeF applies to your Polish operations, contact info@kordeckipartners.com.
Frequently asked questions
Q: Does KSeF apply to a Slovak company that only buys from Polish suppliers and has no Polish VAT registration?
A: KSeF does not require the Slovak buyer to issue KSeF invoices if it has no Polish VAT registration. However, the Slovak company must be prepared to receive and process KSeF-structured invoices from its Polish suppliers. Accounts-payable systems should be updated to capture the KSeF reference number, which is needed to verify the invoice's legal validity under Polish VAT law.
Q: How long does KSeF integration typically take for a mid-size Slovak business with a Polish subsidiary?
A: For a mid-size group using standard ERP software (SAP, Oracle, or Microsoft Dynamics), integration typically takes four to eight weeks. The timeline includes API configuration, FA(2) schema mapping, user-acceptance testing, and staff training. Starting after 1 March 2026 leaves insufficient time before the 1 April deadline.
Q: Is KSeF relevant for Slovak businesses that use IP Box or transfer pricing arrangements in Poland?
A: Yes. Under Polish tax law, intercompany invoices that lack a valid KSeF number cannot be treated as properly issued documents. This directly affects transfer pricing documentation, because the underlying invoices must be legally valid to support arm's-length pricing positions. IP Box claims that rely on qualifying income from intercompany licensing arrangements face the same risk if invoices are non-compliant.
About KORDECKI & Partners
KORDECKI & Partners is a law firm based in Warsaw and Krakow, advising business clients across 30 jurisdictions. Our team combines expertise in Polish and international law with a practical approach to KSeF compliance, VAT advisory, and cross-border tax structuring. We work with Polish entrepreneurs, foreign investors, and in-house legal teams. To discuss your situation, contact info@kordeckipartners.com.
Disclaimer: This publication is provided for informational purposes only and does not constitute legal advice. The information herein should not be relied upon as a substitute for professional legal counsel tailored to your specific circumstances. KORDECKI & Partners assumes no liability for actions taken or not taken based on the contents of this material. For advice regarding your particular situation, please contact info@kordeckipartners.com.