GMB Scotland has said possible proposals for firefighters to cover ambulance crews work when they are not available highlights the lack of cover available in parts of Scotland.
The union has called on Health Secretary Shona Robinson to come clean on what the firefighters pay offer means for Scottish ambulance staff.
GMB Scotland has written to the Minister stating their members in the Scottish Ambulance Service are the ones trained and who have the resources to deal with medical emergencies.
Union leaders have said this offer to the fire brigade staff seems to indicate they will be asked to cover 999 calls when an ambulance is not available.
They said the only reason one would not be available is because they are busy dealing with another incident or siting outside an A&E dept waiting on a bed.
Drew Duffy, senior organiser for public services for GMB Scotland, said: "We are shocked that rather than the Scottish Government meeting with the GMB and talking about how we properly fund the ambulance service they would rather ask another emergency service to pick up the pieces.
"Firefighters have enough to do with being asked to be part time paramedics. Our members are trained and have the equipment to deal with medical emergencies but they need more staff to deal with the demand.
"The people of Scotland expect an ambulance when they call 999 because of a heart attack and that is what they should get.
"I’ve written to Shona Robinson to ask for clarity on what this offer to the fire brigade union and their members mean to our members in the ambulance service."
John Marr, branch secretary for the GMB at SAS, echoed the comments. He said calling on retained firefighters to carry out such duties in the Highlands shows the ambulance service is not properly resourced in the strath and elsewhere.
Mr Marr commented: "The problem is the ambulance service is under resourced and asking another emergency service to pick up he pieces is about manipulating statistics rather than providing the correct service."
The Strathy recently launched a campaign to get shift work introduced at Aviemore ambulance station in place of on call.
Staff based in Aviemore, Grantown and Kingussie can be on call for up to 72 hours taking in weekend periods and are increasingly reporting in as "fatigued" to SAS control.
GMB Scotland says the current staff work patterns in some rural communities including the strath are unsustainable and not fit for the 21st Century.
Local crews are torn between remaining on call when tired so not to let down their home communities and the danger they could pose on the roads and attending incidents when needing sleep.
The union is campaigning for the long-standing on-call system to be replaced by 24/7 cover at the main ambulance stations along the A9 corridor including Aviemore.
They stress the problem can only be solved by the Scottish Government providing more funding to the beleaguered 999 service.