Prudential has announced that it is to close its sites in Belfast, Bristol and Holborn Bars in London.
The site closures are part of a cost-cutting plan
Around 700 jobs will be transferred to sites in the UK and India as part of a £40m annual cost-savings programme set out in December, the insurer said.
It added there would be no compulsory redundancies before the end of 2006, and the relocation programme would take two years to complete.
But the news sparked anger from unions who threatened to strike over the move.
"Amicus is prepared to fight these proposals and we will support our members in any action they choose to take to protect their jobs," Amicus national officer David Fleming said.
"We have told the company in no uncertain terms that we do not accept their proposals and that they are making no attempt to avoid compulsory redundancies."
The union added that it had signed an agreement with Prudential back in 2002 which stipulated that no compulsory redundancies would be made at the firm as a result of offshoring.
Amicus accused the insurer waited until the deal ran out in February before announcing its latest plans, which it claims are "designed to undermine union representation".
"This goes completely against the spirit of our agreements, which have basically been torn up by Prudential," Mr Fleming added.
Prudential said that its Belfast office, which employs about 500 staff, will close by the end of 2007, while the Bristol office, which employs 40, will close by April 2007.
The roles are set to be transferred to sites at Craigforth in Scotland, Derby in the East Midlands and Mumbai in India.
In addition, 120 jobs at the central London head office of Egg, the internet bank owned by Prudential, are to be moved to other London offices.
"Any change that results in redundancies is regrettable," said Prudential chief executive Nick Prettejohn.
"We are consulting with unions and staff organisations and will make every effort to ensure that our people are kept informed and helped through this difficult transition period," he added.
"These reorganisation proposals are an important step in delivering the cost savings that will enable the UK business to continue to build on the strong positions we have established across the financial services market."
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.
Shares in the group edged 1.5 pence higher to 648p shortly after the news.