Jet2 has said it expects to lose £375-£385 million for the year to the end of March in the wake of the Covid-19 pandemic.
In contrast, the leisure airline and tour operator made a profit of £264 million in 2019.
The company currently plans to restart operations in late-June as the UK government reopens the tourism sector.
A statement added: “The continuing successful rollout of vaccines in the UK and the increasing momentum in Europe are both encouraging.
“Additionally, on April 9th, we welcomed confirmation from the Global Travel Task Force that international travel remained on track to reopen in mid-May.
“However, we were disappointed at the lack of clarity contained in the report, in particular the as yet to be populated ‘traffic light’ framework for destinations, and full details and cost of the associated testing regime.”
Jet2 added: “This means the impact and duration of the proposed Covid-19 travel restrictions for summer 2021 remain difficult to determine.”