Originally Posted by
pholling
Depends on the agreement. There are a number of existing agreements between non EU and EU states that pre-date the EU asserting sole negotiating authority. If there is no agreement between the EU as a whole and the country then the old agreements remain in force. Once the EU and that country sign a new agreement it takes precedence. This is what happened when the EU-US Openskies agreement came into force. The old agreements, e.g. the US-NL open skies and Bermuda II all lapsed. If the UK leaves the EU without a deal, or with a deal that doesn't include remaining in the EU negotiated agreements then they loose the rights in those agreements. It remains to be seen if the other countries would honour the previous agreements as these technically ceased to exist when the EU agreement was made.
Note, this is totally separate from the fact that, as it stands, the UK has no international authority to oversee type design and continued airworthiness, which could be far more problematic.
Thanks for clarifying that. The apocalyptic scenario you describe in your final paragraph appears even to me, as a remainer, a very remote possibility. As it would affect not just the EU and the UK but potentially have worldwide consequences, other more powerful nations would in all probability bang the UK and EU's head's together and force some sort of agreement between the two parties.