Switzerland and Britain signed an agreement Monday guaranteeing that flights between the two countries can continue uninterrupted even if London opts to leave the European Union without a deal with Brussels.
"We have managed to ensure a flawless transition, which is in the interest of both of our countries," Swiss Transportation Minister Doris Leuthard said in a statement after signing the deal in Zurich with her British counterpart Chris Grayling.
Switzerland is not a member of the EU, but its relations with Britain are based on a long line of bilateral agreements between Bern and the bloc.
If London and Brussels manage to reach a deal for an orderly Brexit, the bilateral agreements between Switzerland and the EU will continue to apply to Britain through the end of 2020, the statement said.
But in the case of a "no-deal" Brexit, those agreements will no longer apply to Swiss-British relations.
Monday's deal would then kick in on March 30, allowing for a smooth transition and avoiding chaos for travellers between the two countries, it added. There are some 150 flights each day between Switzerland and Britain, mainly through the airports in Zurich, Geneva and Basel, and 25 British airports.
Some 6.7 million passengers flew these routes in 2017.
IATA chief Alexandre de Juniac warned of the "extreme seriousness of what is at stake" and stressed "the huge amount of work that would be required to maintain vital air links".
A technical paper published by the British government indicated that without an overarching Brexit deal, airlines will have to seek two different approvals from each of the rest of the 27 EU members to ensure they have the regulatory framework to fly.
Airlines themselves have warned that their cross-Channel flights may be grounded for lack of insurance if they cannot be sure their pilots' licenses and safety standards are recognised after Britain leaves the EU on March 29.