Aer Lingus is to cut up to 1,500 jobs through redundancy and outsourcing as part of a 74 million euro cost-saving programme.
The cabin crew base at Heathrow is to be closed, but it is believed the company's hub at Belfast International Airport will not be affected.
A deadline of 1 November has been set for the implementation of the plan.
The SIPTU trade union has said it will ballot its members for all-out industrial action.
"It represents a fire-sale of good quality jobs by a management that can see no further than the next quarter's profit and loss sheet," said union secretary Gerry McCormack.
There will be voluntary redundancy or early retirement packages for cabin crew and ground staff in airports, catering and cargo divisions.
Pay freeze
The cabin crew base at Shannon will also close and services from New York, Boston and San Francisco will be staffed with US-based cabin crew from the summer of 2009.
Staff were also told that there would be a pay freeze until the end of 2009. Sick pay entitlements will also be reduced.
New contracts which will introduce performance-related pay and abolish traditional increments will also be brought in.
The company has already reported losses of 22 million euro for this year and is forecasting potential losses of over 100 million euro next year, depending on the cost of fuel.
On Friday, the board of Aer Lingus insisted its cost-cutting programme was necessary to ensure the airline's long-term viability and independence.
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