Luton-based airline EasyJet has warned it will lose up to £845 million this year due to the pandemic.
It will mark the first full-year loss in its history.
The group said it is braced for losses for the year to September 30 of between £815 million and £845 million.
The carrier said losses reached between £295 million and £325 million in its fourth quarter, which were lower than those in the previous three months when planes were grounded during the lockdown.
It said it continued to keep its finances under review and "assess further funding opportunities" amid reports it has warned the government it may need further financial support if the pandemic continues to batter demand.
Johan Lundgren, chief executive said:
EasyJet revealed it flew just 38 per cent of its planned capacity between July and September - normally the peak summer holiday season.
Flying peaked in August and then fell significantly in September when customer demand was hit by changes in government travel guidance and quarantine rules, according to the group.
Over the full year, passenger numbers tumbled by 50 per cent to 48 million.
It is expecting to fly only around 25 per cent of its planned services in the quarter to the end of December.
Holidaymakers are booking at a "very late stage" and demand for destinations is shifting rapidly due to ever-changing quarantine rules, it added.
The group recently agreed a deal with unions to avoid compulsory redundancies as it looks to slash its cabin crew and pilot workforce by up to 30 per cent.
"Discussions with the relevant unions and works councils were constructive and have resulted in greatly increased seasonal and flexible working patterns whileavoiding the need for compulsory redundancies" it said.