Ryanair will cut up to 3,000 jobs and Aer Lingus will axe 900 in a devastating blow to Ireland's aviation industry.
The bulk of the job losses will be among pilots and cabin crew.
Aer Lingus currently employs 4,500 people, while Ryanair employs nearly 17,000 across the group, which also includes Austrian airline Lauda.
Ryanair said its restructuring plans will involve job losses, unpaid leave and pay cuts of up to 20pc.
A number of its bases across Europe will close until passenger traffic demand recovers.
Ryanair chief executive Mich- ael O'Leary will continue to work on 50pc pay for the remainder of the year.
Aer Lingus, part of the IAG group which also owns British Airways, declined to expand on its plans, but said it is continuing to communicate with staff and engage with their representative bodies.
Trade union Forsa said it will be seeking early engagement with Ryanair and will make no public comment until management has formally outlined a detailed position to the union.
Forsa has called on the Government to act swiftly to ensure Ireland has an aviation industry after the Covid-19 pandemic has passed.
The union, which represents workers in all Irish-based airlines, airports and airport authorities, said there was a danger the industry would be so weakened over the coming months it would be unrecognisable by the end of the year.
It said regional airports were particularly vulnerable.
Ryanair said that for the current financial year, which ends next March, it expects to carry fewer than 100 million passengers. That is 35pc below its original target of 135 million, it said.
It is also in talks with Boeing and jet lessors for its Lauda fleet to cut the number of planned aircraft deliveries over the next two years to reflect a "slower and more distorted EU air travel market in a post-Covid-19 world".
"When Ryanair returns to meaningful flying from July, the competitive landscape in Europe will be distorted by unprecedented volumes of state aid from some EU governments and their 'national' airlines," claimed Ryanair.
It said the aid planned for rivals now tops €30bn.
"All this state aid is in breach of EU rules and will distort Europe's level playing field in airline competition for many years," it added.
"Lufthansa, Air France-KLM and Alitalia can now fund many years of below-cost selling."
Ryanair confirmed it expects traffic on reduced flight schedules will be stimulated by "significant price discounting" and what it said will be "below-cost selling from flag carriers with huge state aid war chests".
In a broadside at airports and governments, Ryanair warned they will have to provide additional incentives to airlines.
"These lower fares will require aggressive airport price incentives to encourage passengers to travel, and Ryanair continues to call on EU governments to cut passenger taxes, airport taxes and departure taxes on an industry-wide basis as a better alternative to selective state aid 'doping' for flag carriers," the low-cost carrier said.