Axa has told staff it will embark on a 60-day consultation period over its plans before "proposing the scheme be closed".
Unite, Britain's largest union, has called the decision "appalling and unjustified", and has vowed to challenge the company's plans. It has not ruled out an industrial action ballot to persuade the company to think again.
Axa said it had "worked hard to maintain" its defined benefit scheme over the past few years and had introduced a "number of changes to keep the scheme sustainable".
"This will not affect any past benefits and members of the defined benefits scheme will be invited to join the defined contribution scheme, which has been offered to all new employees since 2003," a spokesman said.
"This allows us to harmonise pension arrangements in a fairer way across our employee base and ensure all of our employees have access to long-term pension provision."
Dominic Hook, Unite national officer, said: "Long-serving staff now face the prospect of having to work an extra five years to get the same level of pension and puts all the investment risk on to the staff.
"Unite will use the 60 day consultation process to challenge these proposals and express our members' anger over these plans."
He added: "The decision by Axa is unacceptable and industrial action will be among the options being discussed with members if Axa refuse to address their anger."
