Union bosses fear the replacement of fire engines with smaller "rapid response" vehicles in Cumbria is a risk to public safety.
The county's Fire and Rescue Service is introducing the new Toyota Hilux off-road vehicles at Staveley and Arnside fire stations from the start of April for a six-month trial.
The fire service insists the move will enhance its capabilities as the new vehicles are more agile and better suited to operating in rural areas than traditional fire engines.
But the Fire Brigades' Union (FBU) says the plans will downgrade stations and reduce firefighter numbers.
Cumbria's FBU Brigade Secretary Ed Burrows wrote to Cumbria County Council outlining his concerns.
He said: "We believe without the austerity we are facing, this would never have come to the table.
"Reduced crews, equipment and water will lead to firefighter and public safety being put at risk.
"No one can predict a serious fire, the public deserves the right resources with adequate firefighters to deal with the incident efficiently and effectively."
Cumbria Fire Service says the new vehicles will only replace fire engines at Staveley and Arnside permanently if the trial proves successful.