Why? EU27 and UK do not agree a withdrawal agreement, or EU or UK Parliament fail to approve the agreement
When? 29 March 2019, when the period set by Article 50 of the Treaty expires, or 31 May 2019 following a short extension to agree emergency measures (e.g. on aviation and security). An extension requires unanimity of member states; new EU Parliament elections are held at the end of May 2019 and this may be regarded as a hard deadline.