If no Brexit deal is reached, British airlines would not be able to fly into EU airports like Charles De Gaulle in Paris | Stephane De Sakutin/AFP via Getty Images Commission to EU27: Prepare airports for a ‘no-deal’ Brexit Countries will have to hire customs officers and expand airports to deal with post-Brexit air traffic.

EU countries should prepare their airports and aviation sector for a "no-deal" Brexit, the European Commission told diplomats at a briefing earlier this month.

The warning came during a June 12 meeting chaired by Filip Cornelis, the director of aviation at the Commission's transport department, attended by EU27 diplomats and representatives of their civil aviation authorities, according to three officials present.

He advised countries to be ready for the possibility of the U.K. crashing out of the EU without a deal on March 29 next year, saying that they should consider their customs handling capacity and prepare for impacts on security, market access, safety regulations and passenger rights.

Brexit negotiators in March agreed on a standstill 21-month transition period during which the U.K. will continue to abide by EU laws. But the transition period is contingent on a broader deal, and with progress in the talks currently stalled over the future of the Northern Ireland border and other issues that are far from certain. The warning of dire consequences for air travel between U.K. and EU destinations will raise the temperature ahead of a meeting of the European Council later this week at which EU leaders will review progress in the Brexit talks.

One official present at the aviation meeting said it is only sensible for governments to prepare for no deal: "It would not make sense for the Commission and the national governments not to prepare for the worst-case scenario."

If the U.K. leaves the EU without an aviation agreement, flights would immediately cease between the islands and the EU27 since EU-issued operating aviation licenses would no longer be valid, and British airlines would no longer have the right to fly to EU countries. The U.K. would also cease being a member of the European Aviation Safety Agency (EASA), which issues the certification and licenses EU aircraft require.

The Commission told officials and diplomats at the briefing about the problems that additional customs checks on cargo would impose on airports under a no-deal Brexit.

Currently, a lot of air cargo from third countries first arrives at the U.K. and is then shipped on to other EU countries, where it doesn't need to be processed by local customs. But in the event of no deal, those shipments would either fly directly into other EU airports or from the U.K., but in either case it would have to be cleared by customs, something that may entail a large increase in staff and infrastructure.

The Commission has in recent months been advising different sectors to make preparations for a no-deal outcome and has drawn up a series of legislative changes that may be required if the U.K. crashes out in March next year. On Friday, Commission President Jean-Claude Juncker told the Irish parliament: "As the clock to Brexit ticks down, we must prepare for every eventuality, including no deal."

Leading aviation industry players have complained about the lack of progress in the Brexit negotiations in their sector. Aircraft giant Airbus warned the British government last week that it is "too dangerous" for the company to continue its U.K. operations without more clarity on the impact of quitting the EU. Its risk assessment concluded that a no-deal scenario would mean losses of €1 billion a week, and in order to prepare for a hard Brexit, it will have to rethink its "footprint in the country."

The British aerospace industry lobby group ADS added that a no-deal Brexit would lead to "severe disruption and interruption of U.K. production" for its member organizations.

During the June 12 meeting, the Commission did not share its own internal plans for dealing with a no-deal Brexit, said another official who was present. "One could get the idea that it is for the stakeholders to save themselves in the worst-case scenario," the official said.

"The approach was different compared to last seminar," the official said. "It could be that the Commission has realized that stakeholders have not done much thus far and is genuinely worried."

The Commission confirmed that the meeting took place, but would not comment on its contents.

A U.K. government spokesperson said ministers are confident they will secure a post-Brexit agreement. “Given the good progress that we are continuing to make in the negotiations we do not expect a no-deal scenario to arise," the spokesperson said.