For the past 20 years, the basic rule has been that any airline based in a European Union country can fly anywhere within the single market whenever it wants.

That access has driven the success of Europe's low-cost carriers. London's EasyJet, Dublin's Rynair and Spain's Vueling have opened new bases and routes that crisscross the continent.

Consumers have benefited from lower fares and more flights.

Now the decision by U.K. voters to quit the European Union puts that all in doubt.

It was no surprise that British airline stocks fell dramatically -- several north of 20% -- after the Brexit vote was announced on Friday.

As part of its divorce proceedings from the European Union, Britain will need to negotiate anew access to that single aviation market. It will also need to find its own place in the so-called Open Skies agreement, which for years has allowed European airlines and U.S. airlines smooth access to one another's markets.

There will be a price to be paid for this. The U.K. may need to accept the supremacy of EU aviation regulations and possibly the freedom of movement of workers. Both conditions would be absolute no-no's for Brexiteers.