Five hundred people who work for Royal Sun Alliance (RSA) in Bristol have been told their jobs are to go by 2010.
Staff were told the closure of the insurance firm's Bristol site was part of several measures made earlier this year designed to save the company £70m.
A spokesman for Unite said the decision to shut the Bristol site would shock the local community.
"Unite is appalled that 500 face redundancy at at time when the job market is so fragile," he said.
"Representatives from Unite will be meeting with the UK chief executive next week to urge him to reconsider the decision to close the Bristol office by June 2010," he added.
Although the company announced 400 full-time job losses this affects 500 people as many posts were job shares. A spokesman for RSA said it is fully committed to supporting its people through the change.
"This includes the provision of comprehensive outplacement services to help them secure new roles - help for writing CVs, liaison with other employers in the area to seek out prospective job opportunities," he said.
Insurance firm worker told of cuts
It said it would continue to have a small trading office in the city employing around 40 people to service its Commercial Brokers department.
RSA is a personal and commercial insurer with 42% of its operation in the United Kingdom. In Bristol the company can trace its routes back centuries to Royal Fire Insurance.
Danny Cox an analyst from Hargreaves Lansdown said financial services played a very important role in Bristol with about 30,000 people employed in that sector.
"This is a devastating shock [for these people]," he said.
"Scottish Widows and Clerical Medical announced yesterday that they have plans to merge."
It comes after Lloyds TSB merged with Halifax Bank of Scotland (HBoS) last year, which has led to a major review of operations that could affect some 5,000 people in Bristol.
BBC West's business correspondent, Dave Harvey, said: Finance has been a huge part of Bristol's economy but the RSA closure shows that nothing is safe in this sector."
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