On 29 March 2019 the deadline for negotiations between the United Kingdom and the EU on the terms of Brexit will expire. The draft withdrawal agreement collapsed on 15 January 2019 due to opposition by the British Parliament. If a withdrawal agreement cannot be reached in the remaining time or the negotiation period is extended, the United Kingdom will exit the EU without any transitional arrangements.

This would have a direct impact on the secondment of employees by a British firm to Germany, and vice versa. So far, affected EU citizens do not need a residence permit or a work permit in Germany or the United Kingdom. From 30 March 2019 this will be difference from a purely legal standpoint—the problem is that there is currently no way to apply for the permit(s). A three-month grace period is currently being discussed in Germany, so that affected employees would have until the end of June 2019 to apply for the same kind of residence permit and work permit that are required for citizens of countries outside the EU.
The social security system for seconded employees is also changing. Up to now, an employee's participation in the social security system of his or her country of origin while working abroad has been documented by an A1 certificate. This avoids both a situation where the employee has no insurance and a situation where the employee is insured twice. If the United Kingdom leaves the EU, the EU-based rules on the coordination of social security systems will no longer apply. These rules will be replaced by the social security convention entered into between Germany and the United Kingdom in 1960. This will apply unchanged and will actually come back into force. The A1 certificate will be replaced by a D-UK-101 certificate. Applications can and should be filed now.
Although the social security convention from 1960 in principle provides comparable protection to that provided by EU law, it is not congruent in terms of coverage. For example, it only applies to the citizens themselves, and does not provide for unemployment insurance. The German federal government therefore presented a draft law on transitional regulations in the areas of employment, education, health, social security and citizenship following the withdrawal of the United Kingdom of Great Britain and Northern Ireland from the European Union. It provides for transitional arrangements so that German and British nationals living and working in the other State at the time of an unregulated Brexit will retain their social security coverage.