The First Union understands the proposal affects 33 fulltime positions and it said the reduction in staff would be "an immense loss for both the community who use the service and the workers affected".
"If anything we need more ambulance professionals on the road," said the union's organiser for ambulance service staff, Neil Chapman.
"No one should be losing a job at St John. If anything, we need more crews in the emergency front line service."
"Currently, outside of our main centres, we're struggling with the unsafe practice of single-crewing ambulances."
The union notes that St John last year told the Government single-crewing is an unsafe work practice.
First Union is urging St John Ambulance to re-deploy the staff from disestablished jobs for double-crewing, "making the job safer for everyone and improving the service in the process".
A St John spokeswoman, when asked why the stated percentage in its staff reduction proposal indicated fewer than the 33 job cuts stated by the union, said: "We are in a period of consultation with staff internally so we cannot confirm final numbers at this stage, particularly as we hope to redeploy a significant number."