A union is balloting its members at Yorkshire Ambulance Service (YAS) in a row over changes to shift patterns.
Unite is asking its 450 members whether they want to take part in strike action in opposition to the introduction of new shifts next month.
It said the shifts were not family-friendly and would leave staff too tired.
Officials also complained the union had not been consulted – but YAS is under no obligation to do so after it withdrew voluntary recognition of the union last February.
It now only recognises Unison, the union which represents the largest number of staff at the trust and has statutory recognition.
Unite said the new shift pattern would be based on 12- hour shifts and could mean paramedics going more than 10 hours without a meal break.
It wants a protected meal break of 30 minutes after six hours.
Terry Cunliffe, regional officer, said: “Our members, who are doing their best for the Yorkshire public in very difficult circumstances, have been under sustained attack by the trust’s hardline management for more than a year.
“The latest erosion in their employment conditions is the demand to work elongated shifts which could mean them working more than ten hours on the trot before managers deign to give them a meal break. This could affect their ability to do their jobs – helping people in distress.”
But YAS has denied all the union’s claims.
David Whiting, chief executive, said: “We refute the latest misleading and factually incorrect claims being made by Unite around staff rest breaks and shift patterns.
“Changes to staff rotas and rest breaks have been agreed after a period of consultation with staff and Unison, following a detailed and comprehensive review of our service which took place throughout 2013.”
YAS said the new rotas would improve the allocation of rest breaks, with a minimum of 30 minutes during a 10-hour shift and 45 minutes during a 12-hour shift.
The ballot will close on January 24.