Brian Pearce, chief economist of the International Air Transport Association (IATA), said cash flow "may not return to positive before the end of the year". At the same time, cash consumption will continue, and even the pace of consumption in Europe is likely to increase.

Pearce said in an online conference that some airlines may have run out of cash. For bailed-out airlines such as Air France-KLM and Lufthansa, a prolonged downturn will increase debt and may need more support.

Airline executives and Wall Street analysts say the resurgence of social blockades has stifled the fragile rebound in bookings. At the time, airlines were preparing for a recovery in the summer.

ForwardKeys, an aviation data expert, said that bookings in Europe in the first half of this year were only 22% of what they were 12 months ago. That compares with 36 per cent in the US and 48 per cent in China.

In response to this situation, airlines have cancelled a large number of flights. Wizz Air, the ultra-low-cost airline that has been expanding its fleet and network during the crisis, has suspended most of its UK routes and expects capacity to fall by 75 per cent in January.

"the blockade puts pressure on demand, and we are adjusting capacity to demand," said Jozsef Varadi, the company's chief executive. "it's going to be a difficult quarter."

Mark Manduca, an analyst at Citi, said the European aviation industry was "about to disappoint". "We believe the risk of recovery continues into the summer because we believe that pre-takeoff tests may dampen demand," he added in a report. " "the slower-than-expected promotion of vaccines to the corporate population could continue to stifle the recovery in corporate demand."

This unstable outlook complicates vital summer flight plans as airlines have to decide months in advance whether to invest in restarting aircraft parked at airports and rehiring staff.

Nearly 3/4 of European routes are now restricted, according to research by UBS, a higher proportion than during the best part of the first wave of health events from March to May last year.

Airlines are disgusted by governments' refusal to abandon measures to conduct pre-flight tests. Ryanair CEO called it "a chaotic measure".