Yvan Thauvette, president of the Union of Veterans' Affairs Employees, said staff are already overburdened, with some case workers handling up to 900 files.
That could rise sharply if the department loses staff as part of Ottawa's promise to cut costs in each department by up to 10 per cent.
"If they intend to cut frontline staff tomorrow, you better watch out because the services to veterans will be lower down," he told reporters in Halifax.
"This exercise, if applied, will have a direct impact on the services provided to veterans."
Thauvette wants Veterans Affairs to be excluded from the ongoing government-wide review aimed at reducing program spending for the next budget by five or 10 per cent in each department.
Veterans Affairs is planning to cut $226 million from its budget in the next two years.
Thauvette said that will likely result in 500 job cuts by 2015, putting more strain on already overloaded workers while degrading services for veterans. He expects that 350 frontline staff who deal directly with the country's roughly 200,000 clients will be lost.
NDP MP Peter Stoffer said veterans already have to wait too long for service from the department after they muddle through complicated paperwork.
He said veterans struggling with physical and mental health issues have trouble wading through the bureaucracy in order to get the services to which they're entitled.
On average, he said it takes at least 16 weeks for a former service member to receive a response from Veterans Affairs and possibly years to resolve a claim.
Cutting staff will only lengthen that process, he said.
"This is the government that wants to balance the books, due to their own fiscal mistakes, on the backs of the heroes of their country," Stoffer said.
"It is absolutely unacceptable."
But a spokesman for Veterans Affairs Minister Steven Blaney said services and benefits for former military members will be maintained despite the cost-cutting.
"We are determined to serve our veterans better and faster and that's why we are improving our service delivery," Jean-Christophe de le Rue said in an emailed statement.
He added that the 500 job cuts would be realized through attrition.
But a 30-year navy veteran said service has done nothing but slow down over the last several years and he fears it will worsen if there are fewer frontline staff helping veterans with complex problems.
"That cut is going to be extremely drastic for veterans because the service is hard to navigate now," said Wayne Finn, adding that he helps other veterans handle their claims.
"I mean, 900 clients for one case manager? That's out to lunch."
Stoffer said job losses could amount to a cut of 40 per cent, if it includes staff at a veterans hospital in Quebec that will be turned over to the province.