EasyJet is considering cutting more than 700 pilot jobs and closing its bases at Stansted, Southend and Newcastle airports, according to union Balpa.
The airline began formal consultation on its proposals on Tuesday after it announced last month it would reduce its workforce by up to 30%.
Balpa general secretary Brian Strutton said up to 727 pilots are at risk of redundancy.
He said: “We know that aviation is in the midst of the Covid crisis and we had been expecting easyJet to make an announcement of temporary measures to help the airline through to recovery.
“But this seems an excessive overreaction and easyJet won’t find a supply of pilots waiting to come back when the recovery takes place over the next two years.”
EasyJet said it would continue flying to Stansted, Southend and Newcastle even if it stopped basing aircraft there.
The airline’s chief executive Johan Lundgren said: “These are very difficult proposals to put forward in what is an unprecedented and difficult time for the airline and the industry as a whole.
“We are focused on doing what is right for the company and its long-term health and success so we can protect jobs going forward.
“Unfortunately the lower demand environment means we need fewer aircraft and have less opportunity for work for our people. We are committed to working constructively with our employee representatives across the network with the aim of minimising job losses as far as possible.”
It emerged on Tuesday that easyJet founder Sir Stelios Haji-Ioannou has seen his shareholding reduced from 33% to 30% following the recent issuing of new shares.
Meanwhile, the European Union Aviation Safety Agency (Easa) has suspended the authorisation of Pakistan International Airlines to operate flights in Europe for six months.
This comes after the country’s aviation minister stated that almost a third of Pakistani pilots cheated on their pilot’s exams.
An Easa spokeswoman said: “This decision was taken due to concerns about the capability of competent authorities to ensure that Pakistani air operators are in compliance with applicable international standards at all times, in view of the recent investigation reported on in the Pakistani parliament which revealed that a large share of pilot licences issued in Pakistan are invalid.”