Following the UK's referendum on withdrawal from the EU ('Brexit') in June 2016, the British government formally notified the EU of its withdrawal decision on 29 March 2017. This triggered a two-year period for negotiations with the EU on the conditions for an orderly withdrawal on 29 March 2019. On 19 March 2018, the parties agreed on a transition period which would run from 29 March 2019 to 31 December 2020 in the event of an orderly withdrawal. During this phase, the UK would remain part of the EU's internal market and customs union (but without codecision rights). Whether the UK's withdrawal from the EU will be orderly and regulated by a withdrawal agreement, and thus include a transition period, will depend on the successful conclusion of negotiations between the EU and the UK.
At present, some points of the withdrawal agreement between the EU and the UK are still open. The parties want to reach an agreement by autumn/winter 2018. Once the negotiations have been concluded, the European Parliament and the British Parliament will have to approve the withdrawal agreement so that it can be ratified by the end of March 2019.