Since the UK’s departure from the European Union on 31 January 2020, the framework governing social security for internationally mobile employees has changed significantly. For employers with staff working across UK and EU/EEA borders, understanding the post-Brexit coordination rules is essential to avoid unexpected social security liabilities, penalties or double contributions. While the fundamentals remain recognisable, the legal basis and some practical outcomes now differ from the pre-Brexit position.

The legal framework: UK–EU Trade and Cooperation Agreement

The primary legal instrument governing social security coordination between the UK and EU is the Trade and Cooperation Agreement (TCA), which took effect on 1 January 2021. The TCA broadly replicates core principles of the EU’s previous coordination regime, namely:

• Employees should be subject to the social security system of only one country at a time.
• Contributions should generally be paid where the work is physically performed.

How do I determine the applicable social security system?

As a general rule, an employee is subject to social security in the country in which they physically carry out their work. For example, a French resident employee working Monday to Friday in the UK will typically be liable to pay UK National Insurance contributions (NICs).

For employers, this means that payroll obligations and employer social security costs are directly linked to where the employee performs their duties, rather than their residence or employer location.

What are the detached worker rules?

Under detached worker rules (also known as posted worker rules), an employee sent temporarily by their employer to work in another country can remain within the home country’s social security system for up to 24 months, provided certain conditions are met. These include:

• The employee must not be replacing another detached worker.
• The employment link with the sending employer must continue.
• The posting must be temporary. When an individual is going to be posted to the other country for more than 24 months from the outset, they are liable for social security in that country from day one.

The UK–EU TCA framework preserves the concept of detached workers. In practice, employers must obtain an A1 certificate (or equivalent) from the home country authority to evidence continued coverage. Failure to secure this documentation can result in dual contribution exposure or compliance challenges.

Unlike the EU’s internal social security coordination rules, an extension from 24 months to up to five years is not possible.

What happens to multi-state workers?

Complex situations arise where employees work in two or more countries on a regular basis. Under the TCA:

• If an employee performs a ‘substantial’ part of their work (generally 25% or more) in their country of residence, they are typically subject to that country’s social security system.
• If not, liability generally falls in the country where the employer is based.

For employers, this creates a need to track working patterns carefully. Remote working arrangements, business travel and hybrid work models can all trigger changes in social security obligations. For example, if it is agreed that a full-time UK employee can work two days a week in Belgium where they normally reside, then they and the employer would be liable to pay Belgium social security contributions.

What are the UK–EEA and UK–Switzerland social security rules?

Social security coordination arrangements apply between the UK and EEA countries (Iceland, Liechtenstein and Norway from 1 January 2024) as well as between the UK and Switzerland (from 1 October 2023) through separate agreements that largely mirror the UK–EU TCA provisions. Employers can generally apply the same principles across these jurisdictions, although local administrative practices may differ.

Is the EU changing social security coordination rules?

The EU has been considering updates to its social security coordination regulations to reflect modern working patterns, including increased remote working. While legislative reform has progressed slowly, practical interim solutions have emerged.

These developments are relevant alongside the UK–EU TCA framework.

Framework agreement on cross-border telework

One of the most significant developments is the ‘multilateral Framework Agreement on cross-border telework’, introduced in 2023 and now adopted by many EU/EEA states (though the UK is not a participant).

Under this agreement:

• Employees who work from home in their country of residence for less than 50% of their working time can remain insured in the employer’s country.
• This relaxes the standard ‘substantial activity’ threshold of 25% that would otherwise shift social security liability to the country of residence.
• It applies only where both relevant countries have signed up to the agreement.

However, take up has been limited, potentially because the framework does not align with the personal income tax treatment.

Future EU proposals

The EU continues to explore broader reforms to its coordination rules to address digitalisation and evolving work patterns.

Proposals to update the current 2010 coordination rules include:

• A new definition of fraud in social security coordination.
• Workers will have to be affiliated to social security in their home country for at least three months before being posted to another EU member state.
• After 24 months of posting, there must be a break of at least two months before another posting.
• Postings must be notified before they happen, apart from business trips (being a maximum of three days in a 30-day period) in all sectors except the construction sector.

Any such reforms may widen the gap between EU internal rules and the UK–EU TCA framework over time.

What are the practical implications for employers?

Post-Brexit, employers face several challenges in relation to social security arrangements. Applications for A1 certificates (or equivalents) are still required, but processes and timelines may vary more significantly between countries, increasing administrative burden. Employers may need to operate payroll in multiple jurisdictions depending on employee working patterns, resulting in payroll complexity. Monitoring requirements are also more intensive, with the need to track employee location and time spent in each country now more critical than ever.

Proactive planning and robust tracking systems are therefore essential to manage compliance risks effectively.

How do I manage post-Brexit social security compliance risk?

While the post-Brexit social security landscape retains familiar principles, the shift from EU regulations to the UK–EU TCA introduces additional complexity and potential divergence. Employers must pay close attention to where work is performed, ensure appropriate documentation is in place and monitor employee working patterns closely.

At the same time, developments within the EU – particularly around remote working – highlight a growing divergence that employers with cross-border workforces must navigate carefully. Early engagement and clear internal policies will be key to managing risk and ensuring compliance in this evolving area.

For more information on how to manage your cross-border social security position after Brexit, please get in touch with Ian Jones, Jo Webber or your usual RSM contact.

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