Ambulance staff in the West Midlands are being balloted over potential strike action.
Members of the union Unison are voting on whether to join a nationwide day of action, on 30 November, over pension changes.
Ravi Subramanian, regional secretary for Unison, said it was the biggest ballot in the trade union's history.
In the event of a strike Mr Subramanian said: "We will absolutely not allow any life to be put at risk."
"We will ensure emergency life and limb cover will be there," he said.
Mr Subramanian said more than a million members were being balloted.
A message from Unison's General Secretary, Dave Prentis, to union members urged them to vote for industrial action to "protect" pensions.
Mr Prentis said ministers were "not listening".
Industrial action could involve public sector workers including non-teaching staff in schools, ambulance staff, support staff with police forces, council workers, and health service workers.
The closing date for the ballot is 3 November.