Posted Workers Documents: A1 Certificate Cross-Checks

Checking posted workers documents on a construction site means comparing four sources against each other: the A1 certificate (social security), the posting declaration filed with the host country, the site or trade card, and the employment contract with payslips. Each document can be valid on its own and still fail the comparison: an A1 that expires before the works end, an employer name that differs between certificate and contract, a worker on site who is missing from the declaration. This article describes what each cross-check compares and what the reviewer does next. It does not claim that a check proves a document is authentic.

Rules described are as of 5 November 2026 and change; confirm procedures on the official sites linked. This is general information, not legal advice.

What does each document prove, and what does it not?

Each document answers one question, and none replaces the others. The most common mistake of main contractors is to treat an A1 as if it covered the whole posting.

Which fields should be compared?

Seven comparisons catch most mismatches on site. The table lists the field, the two sources, the typical gap and the reviewer's next step.

A spelling difference and an expired A1 do not carry the same weight. Log the gap, the answer received and the access decision each time.

Is the host State bound by the A1?

Until the issuing institution withdraws it, an A1 is in principle binding on the institutions of the host State. The Court of Justice confirmed this in Altun (C-359/16, 6 February 2018), while allowing a court to disregard a certificate obtained by fraud after the issuing institution has been asked to review it (CJEU press release).

For a main contractor this means a doubtful A1 is flagged, not rejected unilaterally: raise the mismatch with the employer, record the response, and refer persistent doubt to the competent authorities.

How long can a posting last, and which pay rules apply?

Under Directive (EU) 2018/957, once a posting exceeds 12 months (18 on a reasoned notification), nearly all host-country employment conditions apply, and the host country's rules on remuneration apply from day one (Directive (EU) 2018/957). Member States had to apply it from 30 July 2020.

An A1 can run for up to 24 months, so the two clocks differ. The useful cross-check is cumulative posting time, successive sites included, against the A1 period and the declared period.

What applies on a UK site?

For a UK site, no single UK mechanism equivalent to a host-state posting declaration is verified for this article, so the generic comparison above is what we can state with confidence. The social security side after Brexit is governed by the UK-EU arrangements; the UK government publishes its position on social security coordination and posted workers. Which certificate a worker from another country carries, and for how long, should be read from that guidance and from the document itself. The same four-way comparison (identity, employer, period, declared scope) applies whatever the certificate is called.

If you also run sites in the EU, the host-country declarations differ by State; our editions for France, Germany, Spain and Portugal describe each one.

What do site managers actually ask?

"Is the A1 enough to let someone start?" No. It settles social security only; the host declaration and the site credential are separate.

"The A1 arrives after the worker has started. Now what?" Issuing institutions can issue certificates retroactively, but the contractor should log the delay, request the document and decide whether access continues in the meantime. That decision is internal; no rule makes it for you.

"Does a clean declaration mean the worker is who the paperwork says?" No. A declaration tells you who the employer announced, not who turned up. The match against the identity document remains a separate step.

Where does automation help?

On a site with several subcontractors the table above becomes dozens of comparisons per company. A document-validation tool compares names, birth dates, employer and validity dates across the documents of one file and flags the gaps, with a log of what was checked and when. It shows where documents disagree; it does not say whether a document is genuine.

The construction and public works solution page explains what CheckFile compares in a subcontractor file; see also security and pricing.

Related reading: subcontractor compliance documents in construction and cross-checking temp workers' certifications. The sector overview is in the industry verification guide.

Frequently Asked Questions

Does an A1 certificate replace the host-country posting declaration?

No. The A1 concerns social security and is issued by the sending State's institution, while the declaration is a separate prior notification to the host State's labour authorities. Both are checked, each for its own purpose.

Can a main contractor reject an A1 it finds suspicious?

It cannot invalidate the certificate itself. It can flag the mismatch, ask the employer to explain, and refer the matter to the competent authorities; Altun sets a route that goes through the issuing institution.

What is the maximum duration of an A1 for a posting?

Up to 24 months under Article 12 of Regulation 883/2004. That is separate from the 12- and 18-month thresholds in Directive 2018/957 on employment conditions.

What should be compared first on a site card?

The worker's identity, the employer named and consistency with the posting declaration. A site card does not tell you about trade-specific tickets, which are checked separately.

Does automated checking prove a document is genuine?

No. It compares fields across documents and flags inconsistencies; authenticity of a certificate rests with the institution that issued it.