Tusk has said A50 can be cancelled. Irrespective of the correct legal interpretation, I think if the UK government wanted out of Brexit, they'd get it. This is still a pretty unlikely scenario, but not impossible.

I'm putting that qualification in because, frankly, no-one knows the answer since it's never been put to a court. Moreover, I don't think it will matter. What will matter is whether or not the political will exists to make it happen.