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IANAL but this seems to suggest otherwise
Three. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
http://openeurope.org.uk/today/blog/the-mechanics-of-leaving-the-eu-explaining-article-50/
Will read more and see if I find anything definitive
(also not being confrontational, just wanting to edumacate myself)
No-one knows, as it's never been tested in law.
It's possible that, after 18 months of negotiations, the UK Government decides that any deal it's going to get is unpalatable and decides to remain in the EU. What the EU could or would do in that situation is unknown.