Ambulance crews in Gloucestershire, Bristol and Wiltshire have said they would be prepared to take industrial action over proposed changes to shifts.
The Unison union said the removal of some breaks and other alterations could leave staff without enough rest.
A spokesman said it recognised the need for the service to evolve, but not at the risk of patient and staff safety.
Great Western Ambulance Service said the changes were to make more vehicles and crews available at busy times.
Unison said questionnaires sent to members were not a formal vote for strike action, but indicated what the results could be if an industrial action ballot was issued.
"Ninety-seven percent of responses said they had no confidence in GWAS management and 96% said that they would be prepared to take some form of industrial action short of strike action.
"Ninety-four percent said that, if necessary, they would vote to support strike action," the spokesman said.
Great Western Ambulance said they were not asking staff to work longer and were not cutting salaries
"The changes to shift start and finish times are so that we can keep as many staff on the road at any one time," a statement said.
"The changes we are making are about saving more lives. To do that, we need to ensure we have the trained staff and vehicles available when and where our patients need us."