Prior to the vote, I wasn't paying much attention to Brexit. Like many people, I didn't think Leave would happen. I was wrong, and now I'm trying to play catch up on the issue!
I've seen it said that Article 3(1)(e) of Treaty on the Functioning of the European Union prevents the UK from negotiating trade agreements until it has worked out its exit from the EU. (For example, see the first comment here).
Here is the text of the treaty:
Article 3
1. The Union shall have exclusive competence in the following areas:
...
(e) common commercial policy.
Under this provision, it is clear that an EU member state cannot negotiate with third states in relation to the common commercial policy. But once a member state has decided to leave the EU, and would like to begin negotiating its own commercial policy (i.e., its own trade agreements), does Article 3(1)(e) get in the way?