Two neighbouring fire brigades covering three million people could merge as part of a huge money-saving drive.
Devon and Somerset Fire and Rescue Service is in talks with Avon Fire and Rescue Service over the move to "improve effectiveness".
The two services said they were exploring everything from sharing support services to a full merger.
The Devon and Somerset service faces a deficit of £5.5million over the next two years and unions fear 150 jobs are at risk.
In a joint statement, the services said: "Along with all public sector organisations, both fire and rescue services are facing financial pressures due to reductions in government funding."
Mark Healey, chairman of Devon and Somerset Fire and Rescue Authority, said: "Working with Avon Fire and Rescue Service could provide real opportunities for improving public safety. We recognise that the fire and rescue service has to change and, like others, faces an uncertain financial future."
Terry Walker, chairman of Avon Fire and Rescue Service, said: "During these challenging times we cannot rule out new ways of working to help meet our reduced budgets, while aiming to maintain the much valued service we provide to the public."
The Fire Brigades Union (FBU) said it was "adamantly opposed" to the idea of a merged "mega brigade". It claimed such moves would not improve services.
South West regional secretary Phil Jordan said: "Bigger is not necessarily better for the public or for firefighters.
"Devon and Somerset only merged six years ago, yet it already plans to make cuts to one-in-five frontline firefighter jobs. Clearly the previous merger has not protected frontline services."
Bob Walker, FBU Devon and Somerset chairman, said: "It's a surprise these talks have been going on behind the backs of the communities who will be affected and the firefighters who work in both brigades.
"There has been no assessment of the added risks a mega-brigade will have on the different communities across the South West, who deserve a first-class service."
Chris Taylor, Avon FBU brigade secretary, said: "We want to know how this will impact on response times and what it will mean for our conditions.
"Senior management want to build their mini-empires of influence – we're interested in defending our service and protecting our communities."
