Industrial action ballot dates have been announced for more than 15,000 ambulance workers from 11 trusts in England and Wales in disputes over pay.
Thousands more members of other NHS trusts will also be voted in, the GMB said.
Ambulance workers will start voting on October 24, with the result due at the end of November.
The union said any strike action could take place before Christmas.
Almost 750 paramedics and ambulance staff at West Midlands Ambulance Service are being balloted for strike action to demand a pay rise in line with inflation.
The polls follow advisory votes across all boards in which workers voted strongly to move to a formal strike vote.
The GMB said its members are angry at the Government’s 4% pay award, which it said leaves them facing “another massive pay cut in real terms”.
National officer Rachel Harrison said: “Ambulance workers have just had enough.
“They haven’t been on strike for decades, but they are at the end of what they can take.
“Pay has been systematically reduced for more than 10 years and we now face the worst cost-of-living squeeze in a generation.
“Meanwhile, vacancies are at record highs and we have the worst A&E delays ever, and it’s not even winter flu season yet.
“This is about more than pay and conditions. Cuts and shortages mean GMB members feel they cannot deliver safe standards of patient care.
“Things can’t go on like this, something has to give.”
The ambulance trusts where GMB members will be voted are:
London Ambulance Service NHS Trust
East of England Ambulance Service NHS Trust
East Midlands Ambulance Service NHS Trust
West Midlands Ambulance Service NHS Trust
North East Ambulance Service Bond
Yorkshire Ambulance Service NHS Trust
North West Ambulance Service NHS Trust
South Central Ambulance Service
South East Coast Ambulance Service
Southwest Ambulance Service
Welsh Ambulance Service NHS Trust.