Budget airline easyJet is preparing to axe 727 pilots and permanently close three major UK airport bases as part of a huge restructuring following the global coronavirus pandemic.
The company said hundreds of pilot roles are at risk - with formal consultations starting on Tuesday.
It comes after the low-cost carrier warned it may need to reduce staff numbers by up to 30% back in May, to help "optimise its network and bases as a result of the crisis".
Johan Lundgren, easyJet CEO, 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."
Around 80% of easyJet’s UK pilots currently remain furloughed on the Coronavirus job retention scheme.
"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," Lundgren continued.
"These proposals are no reflection on our people at Stansted, Southend and Newcastle, who have all worked tirelessly and have been fully committed to providing great service for our customers."
The company said it has now informed all affected employees, with staff placed on consultation today.
It comes despite the firm being handed a £600million loan from the UK taxpayer under the Government Covid Corporate Financing Facility.
Union Balpa said it is "shocked at the size of potential job losses" which equates to nearly 1-in-3 of easyJet pilots in the UK.
Brian Strutton, Balpa General Secretary, 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 over-reaction and easyJet won’t find a supply of pilots waiting to come back when the recovery takes place over the next two years.
"EasyJet paid £174m out to shareholders, got agreements to furlough staff to protect cash, got £600m from the Government, has boasted of having £2.4bn in liquidity, and ticket sales are going through the roof so fast they cannot get pilots back off furlough quickly enough – so why the panic? It doesn’t add up. We are meeting easyJet today and we will be fighting to save every single job.
"This is more evidence that aviation in the U.K. is caught in a death spiral of despair and individual airlines are flailing around without direction."
Easyjet - owned by billionaire Sir Stelios Haji-Ioannou - prepares to restart flights to Europe, with plans to serve 75% of its network by August.
From July 1, easyJet will operate around 500 flights per days across its European network, including over 900 flights per week to and from the UK.
Services are due to resume for the first time since all flights were grounded at the end of March.