Unite, Unison and GMB voted to reject proposals to apply up to 25% deductions to sick pay. Officials warned they would ballot workers for industrial action if cuts were imposed.
The proposals were put forward in June as a way of matching the changes made earlier this year to sick pay for other healthcare workers.
"In the course of their work, ambulance staff are exposed to high levels of physical, mental and emotional risk, which makes them vulnerable to illness,"a GMB spokesman said.
"With sickness levels at consistently higher levels than the rest of the NHS, Ambulance Trusts could make more effective savings through addressing the causes of ill health rather than looking to make staff pay for Government cuts."
The NHS Employers organisation said it is "very disappointed" the unions rejected sick pay agreements.
"It is only fair that the same approach to calculating sick pay is used for all staff, including those who work in the ambulance sector," said Dean Royles, chief executive of the NHS Employers organisation.
"These revisions were made to ensure the national pay framework is being responsive to the needs of the service, supporting patient care and improving job security."
NHS Employers said an agreement was stuck earlier this that from March 2013 sick pay would be at the basic rate. No other allowances would be included and the only exceptions were those injured at work and the lowest paid staff.
NHS Employers said this applied to all staff, including ambulance workers.
Royles said: "Ambulance employers have been making operational plans on the basis of the national agreement and have put reasonable proposals forward on how sick pay should be calculated, following a further six months of discussion.
"We anticipate ambulance employers will implement the agreement with effect from 1 September 2013."
Unite head of health Rachael Maskell said: "Management should be called to account for failing in their responsibility to manage sickness and ensure a safer environment for staff to work in.
"Yet cash-strapped employers are now attempting to get our members to pay back their sickness pay coming under the unsocial hours element of their income. This could amount to a substantial loss of income when household bills are soaring."
The unions are expected to hold meetings this week to explore further options before deciding on industrial action.