Five hundred jobs are to be lost at the Prudential Insurance company's Belfast call centre office.
The firm is also closing offices in Bristol and London as part of what it said is a £40m cost-saving exercise.
The company is transferring its operations to three centres in Stirling, Derby and Mumbai in India.
The Belfast office will not close until the end of next year. The trade union, Amicus, said the job losses were a big blow to the Northern Ireland economy.
Kevin McAdam of Amicus, which has pledged to fight the move, said the staff in Belfast were devastated.
"They thought that Prudential in Belfast was one of their big operations," he said.
"Of all the places, we least expected Belfast to close. I will be consulting with my members today about what action we can take."
Amicus has raised the threat of strikes to protect UK workers' jobs.
The union added that it had signed an agreement with Prudential in 2002 which stipulated that no compulsory redundancies would be made at the firm as a result of offshoring.
Amicus accused the insurer of waiting until the deal ran out in February before announcing its latest plans, which it claimed were "designed to undermine union representation".
Chief Operating Officer for the Prudential, Rosie Harris, said she realised it was going to be a "very difficult time for the staff" in Belfast.
"It's not a reflection on the quality of staff and the quality of service that we have provided from here," she said.
"It's more we are looking at how we can have fewer but larger sites."
Prudential said the relocation programme would take two years to complete, and there would be no compulsory redundancies before the end of 2006.
Prudential currently employs 5,765 staff in the UK and 1,115 at a customer service centre in Mumbai. Its Egg business also employs about 2,500 people in the UK.