The federal government's second largest union has backed out of early contract talks over concerns about severance pay.

The Professional Institute of the Public Service of Canada says it's not willing to bargain-away the money its members collect when they retire or quit, indicating severance may be a bigger issue than wage increases.

The union's chief negotiator says getting rid of severance won't keep older workers in the public service.

The union's position comes after the Public Service Alliance of Canada reached a tentative agreement with the federal government earlier this month to scrap severance for employees who either quit or retire. Workers who are laid off will still qualify for severance pay.

The agreement with PSAC includes a 5.3 per cent wage increase over the next three years.