British Airways and Easyjet are among the airlines that are particularly vulnerable to a "no-deal" Brexit, financial services firm Moody's has warned.
The possibility of a no-deal Brexit, in which the UK would crash out of the EU on terms set by the World Trade Organisation (WTO), has been ramping up in recent months, as the UK fast approaches its autumn deadline for finalising a withdrawal agreement with the EU.
According to Moody's, aviation is in even more need of an agreement between the trading partners because it lacks fallback legislation comparable to WTO rules which apply for other industries.
Airlines that would cope the best in the event that the UK did not reach a deal with the EU are those with more flexible operating models and cost structures, greater network diversification and scale, and those that have carried out contingency planning.
Moody's said that while it thought the UK and EU would reach a deal, a no-deal Brexit remained "a threat".
"It could compound an airline's ability to increase yields, raise load factors and generate cost efficiencies in what is still a competitive market environment in Europe," said Jeanine Arnold, Moody's vice president.
"Ultimately it could lead to cash flow and liquidity pressures."
However, it said the strong liquidity of BA, Easyjet and Ryanair should enable them to "weather the financial impact" of a no-deal Brexit, even if flights were disrupted.
Earlier this year Easyjet changed its articles of association to ensure the company would remain EU-owned and controlled after Brexit, thereby allowing it to continue to fly between and within EU countries.
The airline repeated that it was confident flights between the trading partners would continue, even in the event of a no-deal Brexit.
"Consumers, airlines and politicians across the UK and Europe want flights between the UK and Europe to continue," it said.
Ryanair's chief executive Michael O'Leary has taken a different view, telling journalists at a recent press conference that a no-deal scenario was looking more likely. He said the UK had received "no assurances" from the EU that flights could continue after Britain leaves the bloc.
Ryanair also irked investors at its AGM last week when it said it was considering stripping UK investors of their voting rights. It said there was the risk that a hard Brexit would force it to restrict the voting rights of non-EU shareholders so it can remain majority EU-owned and controlled.
Royal London said this contravened the "one share, one vote" principle.