There are rumblings from the Governemnt that the triggering of Article 50 might have to be delayed until Autumn of 2017. That would mean we'd be staying in the EU until late in 2019.

The reason is simple. They can't find enough recruits for the new departments of Brexit and Foreign Trade. Simply put, the departments won't be ready because they had to start from nothing. No one expected this to happen so no one had made any preparations. Getting a Governemnt department up and running takes time.

Source: The Indepentant - Brexit ‘could be delayed until late 2019’ with Whitehall departments not yet ready to trigger Article 50

Source: Metro 15/08/2016