Unite union members among ambulance personnel working in the north of Scotland have voted to start an official dispute with the service.
The workers have concerns that ambulances are not available for emergencies because they are being used for non-emergency patient transfers.
Unite said staff also consistently work over their scheduled hours and suffer from fatigue.
The Scottish Ambulance Service said it had invested in its North Division.
Ninety-five percent of Unite members working in that division who were balloted said they had no confidence in their managers, the union said.
Unite will now register a formal dispute with the Scottish Ambulance Service.
Tommy Campbell, Unite regional officer, said: "Every time we have raised these issues, and every time that a serious incident has been reported in the media, the management of the service have come up with words and spin.
"But our members see these problems every day. They want to deliver a safe, appropriate service to the public across the whole of the north of Scotland, but they have lost confidence in the ability of management to support them.
"We will now raise these concerns through the official disputes procedure."
A spokesman for the ambulance service said: "We have invested significantly in our North Division over the last few years with over 100 extra staff being recruited, 52 of these in the past year, which is an 8% increase.
"There is more we want to do and this is why we have been working closely with our staff, Unite other staff partners and local health boards.
"We will continue to meet with our staff partners including Unite to discuss how we can make the further progress we all want to achieve."