A hitch in national negotiations to reform the fire service could prompt fresh strikes.
Firefighters fought through a long strike
The Fire Brigades Union has warned that the peace deal signed last June could be in "jeopardy".
The executive may support moves at the annual conference next week to break links with the Labour Party.
An FBU source said "Blairite" elements on the employers' side had made last-minute demands at final talks with union leaders.
The employers want to change the overtime agreement and end "stand down time" during two 15-hour shifts.
The executive of the FBU is meeting on Sunday to consider its next step.
Behind the scenes talks are still taking place and it is still possible a deal could be struck.
But the union source stressed that there was much anger among delegates which could spill out at the conference.
There are resolutions calling for a national strike if the pay deal is not met in full from July.
Some brigades are also calling on the union to break the link with the party.
At the moment this is being resisted by its leaders.
In a statement, Councillor Christina Jebb, who chairs the employers' side, said the deal was about modernising the fire service.
She said that firefighters had been offered a substantial pay rise to recognise changing working practices so an automatic right to "stand down time" could not continue.