The UK will have definitely left the EU on 29 March 2019 at the latest.
What needs to be done beforehand to secure your relationships with British clients and suppliers?

It is imperative that all litigation provisions are finalised to determine which law is enforceable and which courts have jurisdiction, or whether mediation can be used, to ensure there is no uncertainty.
British courts will most probably exercise their conflict of laws rule to enforce English law and retain their jurisdiction.
All EU regulations will cease to be enforceable in the UK from 30 March 2019, i.e. in particular EU regulation 1215/2012 on jurisdiction and enforcement of judgments, the Rome I regulation on enforceable law, the regulation on serving documents and the regulation on insolvency.
According to the last state of negotiations as of 8th December 2017, there is a disagreement as to whether a choice of court clause concluded before withdrawal should trigger the application of EU law on recognition and enforcement of judgements in litigation after withdrawal.
However, the other 27 countries remaining in the EU will continue to enforce EU regulations if English law has been provided for in the contract and has been chosen by the parties.
There is uncertainty over the measures the UK will take to deal with international conventions, namely the Rome convention on enforceable law, the old Brussels conventions for proceedings or the Lugano or The Hague conventions for the choice of jurisdiction.
Read more on Eurojuris
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