EasyJet says it has begun consultations on plans to close bases at Stansted, Southend and Newcastle.
It follows an announcement by the airline that it may need to reduce staff numbers by up to a third because of the coronavirus pandemic.
The Unite union said nearly 1,300 crew members faced losing their jobs.
Pilots’ union Balpa said it had been told by EasyJet that 727 of its UK-based pilots were at risk of redundancy.
That is equivalent to one in three of its pilots, Balpa said.
EasyJet chief executive Johan Lundgren said: “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.”
However, Balpa general secretary Brian Stratton said the job cuts were “an excessive over-reaction”.
“EasyJet won’t find a supply of pilots waiting to come back when the recovery takes place over the next two years.”
Easyjet currently has 11 bases in the UK, with 163 aircraft, serving 546 routes.
Even though it is looking at closing Stansted, Southend and Newcastle, it said the airports would remain part of its route network.
The airline said in May that it planned up to 4,500 job cuts as it struggled with the collapse in air travel due to the coronavirus crisis.
Airlines have been hit hard by lockdowns and travel restrictions around the world, with many announcing job cuts.
Even though Easyjet has started to fly passengers again, it does not expect 2019 levels of demand to be reached again until 2023.