An A1 certificate is the document that proves a worker posted from the UK to an EU country, Gibraltar, Iceland, Liechtenstein, Norway or Switzerland stays in the UK National Insurance system and does not have to pay social security contributions in the country where the work is done. Without it, the host country's authorities can demand their own contributions on top of UK ones, which for an employer can add a large slice of salary in unplanned cost. HMRC issues the certificate on application, for employees usually through form CA3822, and the conditions are strict: the posting must be temporary, and an application cannot be made more than 12 months in advance.
What the certificate actually proves
Social security, unlike income tax, is generally payable in one country at a time. The certificate of coverage, issued as an A1 in the countries that use the EU coordination rules, confirms that the worker pays only UK National Insurance contributions while working temporarily abroad, so neither the employee nor the employer owes contributions to the host state's scheme for the covered period. HMRC's guidance on working abroad puts it plainly: in countries with a social security agreement you can get a certificate of coverage to show that you pay National Insurance in the UK and do not need to pay social security contributions in the country where you work. Employers should hold the certificate before the posting starts, because host-country inspectors can ask for it on site.
Applying: CA3822 and who does it
For an employed person, the route is HMRC's CA3822 application, now available as an online service. The form can be completed by the employer, the employee or an agent acting for either, though an agent needs a 64-8 authorisation, and an employer sending staff abroad is expected to have completed form CA3821 first to establish its position. The application covers work in an EU country, Gibraltar, Iceland, Liechtenstein, Norway or Switzerland, and cannot be made more than 12 months before the posting. HMRC will also consider extensions where the stay lengthens, but that requires a justification in writing rather than an automatic renewal, and the underlying condition throughout is that the work abroad is temporary.
Where there is no agreement: the 52-week rule
Outside the countries covered by the coordination rules and the UK's bilateral social security agreements, there is no A1 to apply for. Instead, UK rules require an employee who is ordinarily resident in the UK, was living in the UK immediately before leaving, and is posted abroad temporarily by an employer with a place of business in the UK to keep paying Class 1 National Insurance for the first 52 weeks of the posting, with the employer deducting it from earnings as usual. After that, UK liability normally stops, but the host country's scheme may apply from day one regardless, so double contributions are possible. Workers can also choose voluntary UK contributions to protect their State Pension record while abroad.
Why it matters for remote work and EOR arrangements
The A1 system is built around an employer in one country posting its own employee to another, which is exactly the shape an employer of record arrangement changes. If a UK company's worker is legally employed by an EOR entity in the host country, the worker is typically inside that country's social security system from the start and no UK certificate is involved; if the UK company keeps the employment and sends the person abroad temporarily, the A1 or certificate of coverage route applies. Getting this wrong in either direction creates contribution debt with interest, so the employing entity, the posting dates and the certificate need to be settled before the first payslip, and the paperwork kept for the whole posting.
Questions people ask about a1 certificate
Who applies for the A1 certificate, the employer or the employee?
Either can, and an authorised agent can apply on behalf of both. For posted employees the usual route is HMRC form CA3822, with the employer having completed CA3821 first; HMRC will not accept an application made more than 12 months in advance.
Which countries does a UK A1 certificate cover?
Work in an EU country, Gibraltar, Iceland, Liechtenstein, Norway or Switzerland, under the coordination arrangements that continued after the UK left the EU. Other countries are handled by separate bilateral agreements or, where none exists, by the 52-week rule in UK legislation.
What happens if we post someone without a certificate?
The host country can charge its own social security contributions on the same earnings, and inspectors in several countries routinely ask posted workers for an A1. Applying late is possible, and certificates can be issued to cover a period already worked, but relying on that is a cash-flow and penalty risk rather than a plan.
Does an A1 certificate deal with income tax as well?
No. It only settles which country's social security system applies. Income tax follows separate rules under domestic law and double taxation treaties, so a posted worker can be inside UK National Insurance and still owe host-country income tax at the same time.